Topic 3- Quantitative Chemistry

Question 1

What is the law of conservation?

Answer 1

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

This is because atoms cannot be created or destroyed, although how they are bonded to each other may change.

Question 2

How are chemical reactions represented?

Answer 2

This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

Question 3

What is relative atomic mass?

Answer 3

The average mass of all isotopes of an element.

It is effectively the same as the mass number, which is the larger number found on the periodic table.

Question 4

What is relative formula mass?

Answer 4

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

Question 5

How do you calculate the relative formula mass of magnesium chloride (MgCl2)?

Answer 5

Magnesium chloride contains one atom of magnesium and two atoms of chlorine. Magnesium has a relative atomic mass of 24 and chlorine has a relative atomic mass of 35.5, so the relative formula mass of magnesium chloride is

24 + (2 x 35.5) = 95

Question 6

Why do we calculate the percentage mass?

Answer 6

The percentage mass of an element in a compound is a way of saying what proportion of the mass of the compound is due to atoms of that element.

E.g. The percentage mass of carbon (C) in methane (CH4) is 75%. This means that 75% of the mass of methane is made up of carbon atoms- so if you have 100 g of methane, it will contain 75 g of carbon.

Question 7

How can you calculate the percentage mass of an element in a compound?

Answer 7

If you know the molecular formula of a compound you can work out the percentage mass of a particular element within that compound using this formula:

Percentage mass of an element in a compound

= Ar x number of atoms of that element
_______________________________ x 100
Mr of the compound

Question 8

Example:

A mixture contains 20% iron ions by mass. Given that the only compound in the mixture that contains iron is iron chloride (FeCl2), calculate the mass of iron chloride in
50 g of the mixture.
Relative atomic masses (Ar): Fe= 56, Cl=35.5

Answer 8

1) Find the MASS of iron in the mixture.

The mixture contains 20% iron by mass, so in 50 g there will be 20
50 x —– = 10 g of iron.
100

2) Calculate the PERCENTAGE MASS of iron in IRON CHLORIDE.
Percentage Ar x number of atoms of that element
mass of iron= ——————————————————- x 100
Mr of the compound

56
———————- x 100 = 44.09…%
56 + (2 x 35.5)

3) Calculate the MASS of IRON CHLORIDE that contains 10 g of iron.
Iron chloride contains 44.09% iron by mass, so there will be 10 g of iron in 10 ÷ 44.09
———- = 23 g
100
So you need 23 g of iron chloride to provide the iron in
50 g of the mixture.

Question 9

Exam tip- When you’re doing calculation questions, try not to round your intermediate answers- just round your final answers.

Answer 9

Exam tip- The Ar for any elements you need might be given to you in the question. If not, you can look them up in the periodic table.

Question 10

Why might there appear to be an increase in mass after a reaction has occurred?

Answer 10

If the mass INCREASES, it’s probably because one of the REACTANTS is a GAS that’s found in air (e.g. oxygen) and all the products are solids, liquids or aqueous.

• BEFORE the reaction, the gas is floating around in the air. It’s there, but it’s not contained in the reaction vessel, so you CAN’T account for its MASS.
• When the gas REACTS to form part of the PRODUCT, it becomes contained inside the reaction vessel- so the TOTAL MASS of the stuff INSIDE the reaction vessel INCREASES.

For example, when a METAL reacts with OXYGEN in an unsealed container, the mass of the container INCREASES. The mass of the METAL OXIDE produced EQUALS the total mass of the METAL and the OXYGEN that reacted from the air.

Question 11

Why might there appear to be a decrease in mass after a reaction has occurred?

Tip- remember from the particle model on page 36 that a gas will expand to fill any container it’s in. So if the reaction vessel isn’t sealed, the gas expands out from the vessel, and escapes into the air around.

Answer 11

If the mass DECREASES, it’s probably because one of the PRODUCTS is a GAS and all the reactants are solids, liquids or aqueous.

• BEFORE the reaction, all the reactants are contained in the reaction vessel.
• If the vessel ISN’T ENCLOSED, then the gas can ESCAPE from the reaction vessel as it’s formed. It’s no longer contained in the reaction vessel, so you CAN’T account for its MASS- the total mass of the stuff INSIDE the reaction vessel DECREASES.

For example, when a METAL CARBONATE thermally decomposes to form a METAL OXIDE and CARBON DIOXIDE GAS, the mass of the reaction vessel will DECREASE if it isn’t sealed. But in reality, the mass of the METAL OXIDE and the CARBON DIOXIDE produced will EQUAL the mass of the METAL CARBONATE that decomposed.

Question 12

What is the Avogadro constant?

Answer 12

If we were to place a single atom onto a laboratory scale it would read 0 grams, this would happen if we placed 100 or even a million atoms on a scale.

In chemistry experiments, we are often weighing things out, and clearly, if we want to be able to measure the mass of a substance, we need to measure many more atoms than just a million… but how many atoms exactly?

If you place 602000000000000000000000 atoms of carbon onto the scale, the reading will show exactly 12.00g-this number 12 is exactly equal to its mass number of carbon.

                                                                                23 So Avogadro's constant is just a number, 6.02 x 10

Question 13

What is Avogadro’s constant also known as?

Answer 13

The number is also called the mole.

Question 14

What are chemical amounts measured in?

What is the symbol for this unit?

Answer 14

Chemical amounts are measured in moles. The symbol for the unit mole is mol.

Question 15

What is the mass of one mole of any substance?

Answer 15

The mass of one mole of a substance in grams is numerically equal to its relative formula mass.

Question 16

What is a mole?

Answer 16

The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole.

Question 17

How many particles are in one mole of any substance?

Answer 17

23

A mole is 6.02 x 10 particles.

Question 18

What are moles used to describe?

Answer 18

Moles can be used to describe many things, including atoms, molecules, ions, electrons and chemical formulas.

Question 19

How can you calculate the number of moles in a given mass of a substance?

Answer 19

Using this formula:
Mass in g (of element or compound)
Number of moles= ____________________________
Mr (of element or compound) or Ar
(of element)

Or remember…

n= m ÷ Mr

n=moles
m=mass
Mr= atomic mass (Ar) or molecular mass (Mr)

You can remember this formula as Mass=Mr x moles

Question 20

Example:

How many moles are there in 42 g of carbon?

Answer 20

The Ar of carbon is 12, so the number of moles in 42 g of carbon is:

Moles=mass ÷ Ar = 42÷12 = 3.5

Question 21

How can you calculate the masses of reactants and products?

Answer 21

The masses of reactants and products can be calculated from balanced symbol equations.
Chemical equations can be interpreted in terms of moles. For example:
Mg + 2HCI   MgCI2 + H2

Question 22

What can you learn from balanced symbol equations?

Answer 22

E.g.
Mg + 2HCI   MgCI2 + H2

Shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.

Question 23

How can you calculate reacting masses?

Answer 23

You can use the idea of conservation of mass to work out the mass of individual reactants and products in a reaction.

E.g. 30 g of magnesium is formed from 18 g of magnesium. What mass of oxygen reacted?

The total mass of the product is 30 g, so the total mass of the reactants must be 30 g. The mass of the magnesium is 18 g, so the mass of the oxygen must be 30 - 18 = 12 g.

Question 24

How can you calculate amounts of substances in equations?

Answer 24

The masses of reactants and products can be calculated from balanced symbol equations.

For example, if hydrogen gas is completely combusted in 64g of O2, how many grams of H2O will be produced?

2H2 + O2 ͢ 2H2O

2 x 4= 8 16 x 2=64 g 1 x 4 + 32= 36 x 2 = 72 g

Question 25

How can you calculate amounts of substances in equations?

Answer 25

The masses of reactants and products can be calculated from balanced symbol equations.

For example, if hydrogen gas is completely combusted in 64g of O2, how many grams of H2O will be produced?

2H2 + O2 ͢ 2H2 O

1 x 4= 4 16 x 2=64 g 1 x 4 + 32=36 x 2 =72g

Question 26

How can you balance equations using reacting masses (moles)?

Answer 26

If you know the masses of the reactants and products that took part in a reaction, you can work out the balanced symbol equation for the reaction by:

1. Divide the mass of each substance by its relative formula mass (Mr) to find the number of moles.
2. Divide the number of moles of each substance by the smallest number of moles in the reaction (that you worked out in step one).
3. If any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers.
4. Write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas.

Question 27

What is a limiting reactant?

Answer 27

Reactions don’t go on forever- you need stiff in the reaction flask that can react. If one reactant gets COMPLETELY USED UP in a reaction before the rest, then the reaction will STOP. That reactant is called LIMITING.

Question 28

How long does it take for a reaction to stop?

Why are reactants added in excess?

Answer 28

When some MAGNESIUM CARBONATE is placed into a beaker of HYDROCHLORIC ACID, you can tell a REACTION is taking place because you see lots of BUBBLES OF GAS being given off.

After a while, the amount of fizzing SLOWS DOWN and the reaction eventually STOPS…

• The reaction stops when all of one of the reactants is USED UP.
• Any other reactants are in EXCESS. They’re usually added in excess to MAKE SURE that the other reactant is used up.

Question 29

What is the reactant that is used up in a reaction called?

Answer 29

The reactant that’s USED UP in a reaction is called the LIMITING REACTANT (because it limits the amount of product that’s formed).

Question 30

How much product is formed during a reaction and why?

Answer 30

The amount of product formed is DIRECTLY PROPORTIONAL to the amount of LIMITING REACTANT.

For example, if you HALVE the amount of limiting reactant the amount of product formed will also HALVE. If you DOUBLE the amount of limiting reactant the amount of product will DOUBLE a(as long as it is still the limiting reactant).

This is because if you add MORE REACTANT there will be MORE REACTANT PARTICLES to take part in the reaction, which means MORE PRODUCT PARTICLES.

Question 31

How can you calculate the mass of a product?

Answer 31

You can calculate the mass of a product formed in a reaction by using the mass of the limiting reactant and the balanced reaction equation.

1. Write out the BALANCED EQUATION.
2. Work out the RELATIVE FORMULA MASSES (Mr) of the limiting reactant and product you want yo find the mass of.
3. Work out HOW MANY MOLES there are of the LIMITING REACTANT.
4. Use the balanced equation to work out HOW MANY MOLES there’ll be of the OTHER substance. In this case, that’s how many moles of PRODUCT will be made of this many moles of REACTANT.
5. Use the number of moles to calculate the MASS.

Question 32

What are solutions?

Answer 32

Solutions are mixtures that contain one substance (the solute) dissolved in another substance (the solvent). The easiest way to describe the amount of solute in a solvent is by its concentration.

Question 33

Tip!- If a reaction is said to be carried out in air, then any gases in the reaction that are found in air (e.g. oxygen) will be in EXCESS (unless you are told otherwise).

Answer 33

Tip- A SOLUTE is a substance that is dissolved in a liquid (the solvent). When a solute is dissolved un a SOLVENT they form a SOLUTION.

Question 34

What is concentration?

Answer 34

The concentration of a solution is the amount of solid material (solute) dissolved in a certain volume of liquid (solvent). The more solute dissolved in the liquid, the more concentrated that solution will be.

Question 35

What is the concentration of a solution measured in?

Answer 35

The concentration of a solution can be measured in two ways one of them being mass per given volume of solution, eg grams per dm3 (g/dm3).

Question 36

Explain the relationship between the mass of solute, the volume of solution and the concentration of solution…

Answer 36

The greater the mass of solute in a given volume, the higher the concentration. So if you take a solution and dissolve more of the solute in it, the concentration will increase. If you do anything to increase the volume (e.g. adding more solvent) without also increasing the amount of solute then the concentration will decrease.

Question 37

Are the measurements from using a formula always exact?

How are measurements made more reliable?

Answer 37

All measurements have some uncertainty to them.

Experiments will often be repeated, and then an average (mean) of any repeated measurements will be calculated. The range of the results can also be found, and can be used to give you an idea of how uncertain the mean value is. The greater the range (i.e the more distributed the results), the higher the uncertainty will be.

Question 38

What is the other way the concentration of a solution can be measured?

Answer 38

In chemistry, the concentration of a solution is usually measured in MOLES of a substance per CUBIC DECIMETRE of water. So, a solution with a concentration of 1 mole per dm3 (or 1 mol/dm3) has 1 mol of stuff in 1 dm3.

Question 39

What is the formula that links the concentration of a solution in mol/dm3?

Answer 39

amount of solute (mol)
Concentration (mol/dm3)=_______________________
volume of solution (dm3)

You can remember this formula as c = n ÷ v

or n = c x v

Question 40

How can you find the amount of solute in a solution?

Answer 40

If you know the concentration of a solution in mol/dm3, you can rearrange the equation to find the amount of solute in a given volume.

Example- what mass of sodium hydroxide is there in 0.450 dm3 of a 0.600 mol/dm3 solution?

First find the number of moles in the volume of solution.

number of moles= concentration x volume =

                                         0.600 x 0.450=0.270 mol Then find the mass of this amount of moles using  mass=Mr x moles

mass of NaOH= 40 x 0.270= 10.8 g

Question 41

How can you convert between concentrations in mol/dm3 to g/dm3 ?

Answer 41

You can convert from a concentration in g/dm3 to a concentration in mol/dm3 using the equation:

 Mass = Mr x moles

Tip- grams per dm3 is the same as grams per litre.

Exam tip- always remember to check what UNITS the data you’re given is in. If you’re given volume in cm3 you need to convert it into dm3 before putting the numbers into the equation.

Question 42

What 3 formulas do you need to know in order to find the concentration of a solution in g/dm3 and mol/dm3 ?

Answer 42

1) m = c x v
2) m = n x Mr
3) n = c x v

Question 43

What volume in dm3 do gasses occupy?

Answer 43

At the same TEMPERATURE and PRESSURE, EQUAL NUMBERS OF MOLES of any gas will occupy the same VOLUME.

Question 44

State the conditions of room temperature and pressure…

Answer 44

Room temperature is 20 ℃ and pressure is 1 atm.

Question 45

What is the volume of gas at room temperature and pressure?

Answer 45

At ROOM TEMPERATURE and PRESSURE one mole of ANY gas occupies 24 dm3.

Question 46

What is the formula to calculate the volume of gas?

Answer 46

Mass of gas
Volume of gas = ——————- x 24
Mr of gas

Question 47

How can you calculate volumes of gases in reactions?

Answer 47

For reactions between gases, you can use the volume of one gas to find the volume of another. You just need to look at the molar ratio of the gases in the balanced equation- this will be the same as the ratio of the volumes of the gases in the reaction.

Question 48

Example1- How much carbon dioxide if formed when 30 dm3 of oxygen reacts with carbon monoxide?
2CO + O2 —–> 2CO2

Answer 48

How much carbon dioxide if formed when 30 dm3 of oxygen reacts with carbon monoxide?
2CO + O2 —–> 2CO2

From the balanced equation, 1 mole of O2 reacts to form 2 moles of CO2. This means that the ratio of the volumes of O2 : CO2 is 1:2.

So 30 dm3 of O2 reacts to form (2x30 dm3) = 60 dm3 of CO2

Question 49

Example 2- what volume of hydrogen reacts to form 18 dm3 of ammonia?
N2 + 3H —–> 2NH3

Answer 49

what volume of hydrogen reacts to form 18 dm3 of ammonia?
N2 + 3H —–> 2NH3

From the balanced equation, 2 moles of ammonia are formed from 3 moles of hydrogen. This means that the ratio of volumes of H2: NH3 is 3:2.

So 18 dm3 of NH3 is formed from (3÷2 x 18 dm3) = 27dm3 of H2.

Question 50

How many products are made during a reaction?

Answer 50

A lot of reactions make MORE THAN ONE PRODUCT. Some of them will be USEFUL, but others will be WASTE.

Question 51

What is atom economy?

Answer 51

The ATOM ECONOMY (or atom utilisation) of a reaction tells you how much of the MASS of the reactants is wasted when manufacturing a chemical and how much ends up as useful products is.

Question 52

What is the equation used to find the atom economy of a reaction?

Answer 52

mass of desired product
Atom economy = ————————————– x 100
total mass of all products

Question 53

Why is atom economy important?

Answer 53

It is important for sustainable development and for economic reasons to use reactions with a high atom economy- this will lead to less wastage of atoms.

Question 54

What does it mean if you have 100% atom economy?

Answer 54

100% atom economy means that ALL the atoms in the reactants have been turned into USEFUL (desired) PRODUCTS. The HIGHER the atom economy the ‘GREENER’ the process.

The more products there are, the lower the atom economy is likely to be.

Question 55

Why are low atom economies bad?

Answer 55

Reactions with low atom economies aren’t usually profitable. They need more raw materials to produce a certain amount of product compared to reactions with high atom economies. They also produce more waste than reactions with high atom economies. Raw materials can be expensive to buy and waste products can be expensive to remove and dispose of responsibly.

Question 56

What are the solutions to making low atom economies more sustainable?

Answer 56

The best way around these problems is to find a use for the waste products rather than just throwing them away. Since there’s often more than one way to make the product you want, you could also find a reaction that has a similar atom economy, but gives useful ‘BY-PRODUCTS’ rather than useless ones.

Question 57

What are the economic advantages of high atom economy?

Answer 57

• You make a greater yield of product.
• They use fewer raw materials than reactions with low atom economies to produce the same yield.Many raw materials will run out eventually, so it makes sense sense to use them efficiently so they last as long as possible.
• They produce less waste- this is better for the environment as waste chemicals are often harmful and can be difficult to dispose of in a way that minimises their harmful effects.

All of this means that high atom economy processes tend to be more SUSTAINABLE than low atom economy processes.

Question 58

What are the environmental advantages of high atom economy?

Answer 58

Reactions with a high atom economy use fewer raw materials than reactions with low atom economies. Many raw materials will often run out eventually, so it makes sense to use them efficiently so they last as long as possible. Reactions with a higher atom economy also produce less waste. This is better for the environment as waste chemicals are often harmful and can be difficult to dispose of in a way that minimises their harmful effects.

Al of this means that high atom economy processes tend to be more SUSTAINABLE than low atom economy processes. Doing something sustainably means using up as little of the Earth’s resources as you can and not putting loads of damaging chemicals into the environment- in other words not messing things up for the future.

Question 59

What other factors are considered when choosing which reaction to use to make a certain product?

Answer 59

Things such as YIELD, the RATE of the reaction and the POSITION OF EQUILIBRIUM for reversible reactions also need to be thought about. A reaction with a low atom economy, that produces USEFUL By-PRODUCTS, might also be used.

Question 60

What is yield?

Answer 60

The AMOUNT OF PRODUCT you get is known as the YIELD.

Question 61

What does the amount of yield depend upon?

Answer 61

• The amount of reactant used. The more reactant you start with, the higher the ACTUAL YIELD will be.
• The METHOD you use. As different reactions (to make the same product) will have different yields, even if the theoretical yield is the same.

Question 62

What is the theoretical yield?

Answer 62

The THEORETICAL YIELD of a reaction is the amount you would get if ALL the REACTANTS FORMED PRODUCTS, and NONE of the products were LOST.

Question 63

How can you calculate the theoretical yield?

Answer 63

The theoretical yield can be calculated from the balanced reaction equation, or from the masses of the reactants.

Question 64

What is the percentage yield?

Answer 64

The percentage yield is a comparison between the theoretical yield and the yield you actually get.

It is always somewhere between 0 and 100%.
A 100% yield means that you got all the product you expected to get .
A 0% yield mean that no reactants were converted into product, i.e. no product at all was made.

Question 65

How can you calculate the percentage yield?

Answer 65

mass of product actually made (g)
Percentage yield =————————————————- x 100
maximum theoretical mass of product (g)

Question 66

Why should industrial processes have a high percentage yield?

Answer 66

Industrial processes should have as HIGH a percentage yield as possible to REDUCE WASTE and REDUCE COSTS.

Question 67

Can you get a 100% percentage yield?

Answer 67

Even though no atoms are made or destroyed in reactions, in real life, you never get a 100% yield.

Question 68

Why are percentage yields always less than 100%?

Answer 68

Some product or reactant ALWAYS gets lost along the way. How this happens depends on WHAT SORT OF REACTION it is and what apparatus is being used. There are three common problems.

Question 69

What are the 3 common ways in which product or reactant is lost during a reaction?

Answer 69

1. The reaction is reversible.
2. Product is lost when it’s separated from the reactants.
3. Side reactions.

Question 70

What’s a reversible reaction?

And how can it affect the yield?

Answer 70

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants.

This means that the reactants will never be completely converted to products because the reaction goes both ways. Some of the products are always reacting together to change back to the original reactants. So the yield will NEVER be 100%.

Question 71

Give an example of a reaction which is reversible…

Answer 71

For example, in the Haber process, at the same time as the forward reaction is occurring, the reverse reaction is also happening.

This means the reaction NEVER goes to COMPLETION.

Question 72

How do side reactions affect yield?

Answer 72

Sometimes there can be unexpected reactions happening, known as side-reactions. For example, the reactant might react with gases in the AIR, or IMPURITIES in the reaction mixture, so they end up forming EXTRA PRODUCTS other than the ones you want.

Question 73

How is product lost when it’s separated from the reactants?

Answer 73

When you FILTER A LIQUID to remove SOLID PARTICLES, you nearly always lose a bit of liquid or solid.

• If you want to KEEP THE LIQUID, you’ll lose the bit that remains with the solid and filter paper (as they always stay a bit wet).
• If you want to KEEP THE SOLID, some of it’ll get left behind when you scrape it off the filter paper.
  You’ll also lose a bit of material when you TRANSFER it form one container to another-even if you manage not to spill it. Some of it always gets left behind on the INSIDE SURFACE of the old container.

Question 74

Explain why a particular reaction pathway is chosen to produce a specified product given appropriate data such as atom economy (if not calculated), yield, rate, equilibrium position and usefulness of by-products…

Answer 74

1. ATOM ECONOMY
   Reactions with higher atom economies produce less waste and need less raw materials. This generally makes them cheaper and more sustainable to run.
2. YIELD
   Ideally, reactions have a high percentage yield. This will mean less of the reactants are wasted, so the reaction will be cheaper. It will also be more sustainable, as less waste will be made and less raw material will be used.
3. RATE
   Th RATE of reaction is a measure of how quickly the reactants react together to form the products. It’s no good having industrial reaction that takes forever– you want your product to be made at a reasonable rate.
4. EQILIBRIUM POSITION
   In a REVERSIBLE REACTION, the products react together to reform the reactants. In a sealed container, Reversible reactions will eventually reach a point where the rate of reaction between the reactants is the same as the rate of the reaction between the products, and so the amount of reactant and product in the container doesn’t change. This is known as equilibrium. The amount of reactants and products at equilibrium depends on the reaction conditions. Being able to control the position of equilibrium to maximise the amount of product is important for making an industrial process profitable. Reversible reactions used in industry should have an equilibrium position where the amount of products formed is high and the amount of reactants left is low.
5. USEFULLNESS OF BY-PRODUCTS
   By-products are things produced by a chemical reaction that aren’t the desired substance. Often, by-products are waste, but sometimes the by-Products can be used for something else. So, one way to reduce the waste in a reaction is to choose one that forms useful by-products as well as the intended product.