Quantatative chemistry

Question 1

What is the difference between relative atomic mass and relative formula mass ?

Answer 1

Relative atomic mass (Ar) is of an element and can be found on the periodic table
Relative formula mass (Mr) is of a compound and you add together the sum of all of the (Ar) of each element
eg C has a relative atomic mass of 12
CO2 has a relative formula mas of 12+16+16 = 44

Question 2

How do you find the % mass of an element in a compound?

Answer 2

1.Use a periodic table to find the Ar of the element and the Mr of the compound.
2. Ar/Mr x 100 will give the percentage

Question 3

What is a mole in chemistry?

Answer 3

It is the amount of a substance which contains the Avagadro constant number of particles.

Question 4

What is the Avagadro constant?

Answer 4

It is 6.02 x 10 to the power 23

Question 5

What is the relationship between moles and the relative atomic mass?

Answer 5

One mole of any substance weighs the same as it’s relative atomic mass.
Eg one mole of Carbon weighs 12g
one mole of O2 weighs 16 x 2 = 32g

Question 6

How do you calculate the number of moles given the weight of a substance

Answer 6

1. Calculate the Ar or Mr value from the periodic table
2. Divide the mass by the Mr or Ar value

Eg how many moles are there in 112g of carbon monoxide
1. Mr of CO = 12 + 16 =28
2. 112/28 = 4 moles

Question 7

Nov 2021 C1 Higher
Element R is extracted from its oxide by reduction with hydrogen.
The equation for the reaction is:
3 H2 + RO3 → R + 3 H2O
The sum of the relative formula masses (Mr ) of the reactants (3 H2 + RO3) is 150
Calculate the relative atomic mass (Ar ) of R.
Relative atomic masses (Ar ): H = 1 O = 16 [2 marks]
Identify element R [1 mark]

Answer 7

6 H atoms = 6 x 1 = 6
3 O atoms = 3 x 16 = 48

150 - 6 - 48 = 96 therefore the relative atomic mass of element R is 96

R must be molybdenum

Question 8

What is meant by the term conservation of mass

Answer 8

During a chemical reaction atoms are not destroyed or created.
This means that there are the same number of each type of atom on both sides of an equation.
It also means that the mass remains the same- therefore conserved.

Question 9

How could you explain an increase of mass in a reaction occurring in an unsealed vessel

Answer 9

Mass will only be increased if there is a gas involved in the reaction.
The mass of the gas floating in the vessel before the reaction is not contained within it so cannot be measured.
The gas reacts and becomes a part of the product. It is then contained in the vessel so becomes part of the total mass which increases

Question 10

How could you explain a decrease of mass in a reaction occurring in an unsealed vessel

Answer 10

It cn occur during a thermal decomposition reaction
Before the reaction all of the reactants are contained in the vessel.
During the reaction one of the products is a gas. The gas can escape from the vessel therefore the mass of the products left in the vessel after the reaction decreases.

Question 11

How many atoms are there in a mole of any substance?

Answer 11

6.02 x 10 to the power 23

Question 12

What is the equation you would use for a question on mass and number of moles?

Answer 12

Mass (g) = no of moles x relative formula mass (Mr)

Question 13

How many moles are there in 4g of CH4

Answer 13

Mr of CH4 = 12+ 1+1+1+1 = 16
no of moles = mass/Mr
no of moles = 4/16 = 0.25 moles

Question 14

How many atoms of each element are there in the molecule Ca(HCO3)2

Answer 14

Ca 1
H 2
C 2
O 6

Question 15

How can you tell how many moles of each substance there are in an equation?

Answer 15

The number before the formula of each reactant and product is the number of moles.

Question 16

State how many moles there are of each of the reactants and products in the formula

6Co2 + 6H20 —–> C6H12O6 + 6O2

Answer 16

6 moles of carbon dioxide
6 moles of water
1 mole of glucose
6 moles of oxygen

Question 17

Finding an equation using the masses of the reactants and products involves 4 steps, what are they.

Answer 17

1. Calculate the Mr for each product and reactant
2. Divide the mass by the Mr to find the number of moles.
3. Divide all of the answers by the smallest one then manipulate the number of moles using division or multiplication to make them all whole numbers keeping the ratios the same.
4. When they are whole numbers you have the equation

Question 18

What is meant by excess in terms of reactants in a chemical reaction.

Answer 18

it is common for more of one reactant than is needed is used so that all of the other reactant is used up. This is called the excess.
Eg when fuel is burned an excess of oxygen is used to ensure it is all burnt.

Question 19

Explain the term limiting reactant

Answer 19

It is the reactant in the reaction which determines the amount of products formed. The other reagants are in excess so will not all react.

Question 20

the equation for the reaction between magnesium and sulphuric acid is

Mg + H2SO4 ——-> MgSO4 + H2

If 5 moles of magnesium react with 7 moles of sulphuric acid which is in excess and which is the limiting factor?
How many moles of each product are formed?

Answer 20

Because the ration of Mg to H2SO4 are the same 5 moles of Mg react with 5 moles of H2SO4. Therefor the amount of magnesium is the limiting factor and the sulphuric acid is in excess.

The formula will be 5Mg + 5H2SO4
therefore the products are 5MgSO4 +5H2

Question 21

How do you calculate the mass of a product formed using the mass of the limiting reactant?

Answer 21

1. Write out the balanced equation
2. Work out the Mr for each of the reactants and products
3. Calculate the number of moles in the substance you have been given the mass of.
4. Using the equation you will be able to calculate how many moles of the product there are as the ratio must always remain the same.
5. Multiply number of moles of the product by it’s Mr to find the mass of the product

Question 22

What is the formula for calculating concentration of a solution?

Answer 22

No of moles = Concentration x Volume

Question 23

What are the si units for concentration

Answer 23

mol/dm3

Question 24

How to work out concentration of a substance given the concentration of another within the equation

Answer 24

You will be given a formula and the volume and concentration of one of the reagents and the volume of another and asked for it’s concentration
1. Using the formula find the number of moles you have of the substance (conc x Vol)
2. Keeping the same ratio write in the number of moles of each substance onto your equation
3. You now know the volume and number of moles so you can find the concentration (no of moles/vol)

Question 25

How do you convert concentration in mol/dm3 to g/dm3

Answer 25

You know how many moles you have in one dm3 from the concentration
1. work out the Mr of the substance
2. Use the formula mass in g = moles x Mr.
That gives the mass in one dm3 so g/dm3

Question 26

How do you convert cm3 to dm3

Answer 26

Divide by 1000

Question 27

What is the formula for atom economy?

Answer 27

atom economy = Mr of desired product/ Mr of all products x 100

Question 28

What is meant by the term atom economy?

Answer 28

It is the percentage of reactants which form useful products.

Question 29

Steps for calculating atom economy

Answer 29

1. Work out the Mr for all of the reactants
2. Work out the Mr of the desired product
3. Divide the Mr of the desired product by Mr of all reactants and multiply by 100

Question 30

Explain how low atom economy is bad for the environment

Answer 30

1. They use up resources quickly
2. They make a lot of waste materials which need disposing of responsibly
3. More raw materials increase the likelihood of unsustainability

Question 31

Explain how low atom economy can affect profitability.

Answer 31

1. More raw materials used therefore higher costs
2. Cost in disposing of unwanted waste products responsibly.
3. The more products available the higher the profits.

Question 32

Describe the relationship between number of products and the atom economy in a reaction.

Answer 32

1. If there is only one product then atom economy is 100%
2. If there ar more than one product and both products are useful then atom economy is also 100%
3. If there are any products which are not useful then atom economy is reduced.

Question 33

Identify 4 factors which may affect the decision to choose which reaction to use to make a product.

Answer 33

1. Atom economy
2. Yield
3. Rate of reaction
4. Position of the equilibrium in a reversible reaction.

Question 34

What is the difference between theoretical yield and actual yield?

Answer 34

Theoretical yield is the amount of product which you should get and actual yield is the amount you do get.

Question 35

What is the equation to calculate percentage yield?

Answer 35

% yield = actual yield/theoretical yield x 100

Question 36

Identify 3 reasons for achieving less than 100% yield

Answer 36

1. Not all of the reactants react to make the product.
2. There may be side reactions such as with impurities or gases in the air
3. Product is lost when it is separated from the reaction mixture or when transferring it between containers.

Question 37

RTP is used when calculating volumes of gases, what does it stand for and what is the standard used.

Answer 37

RTP means room temperature and pressure. The standard used is 20 degrees C and 1 atmosphere of pressure.

Question 38

What is the volume of one mole of gas at rtp

Answer 38

24dm3

Question 39

How can you calculate the volume of gases in experiments?

Answer 39

If you are given the volume of one gas in an equation you can calculate the volume of any others from the ratio of the moles.
Eg
2CO + O2 ——> 2CO2
How much CO2 is formed when 30dm3 of O2 reacts with carbon monoxide?
the ratio of oxygen to carbon dioxide in the equation is 1:2
therefore the ratio of gas is 30:60 and the answer is 60dm3

Question 40

What are the steps for calculating concentration?

Answer 40

1. Check the volume of solvent is in dm3 if it is in cm3 then divide it by 1000
2. Divide the mass or number of moles of solute by the volume of solvent
3. Your answer will be in g/dm3 if it was mass and mol/dm3 if it was moles.

Question 41

How do you calculate the volume of a gas at rtp from the mass?

Answer 41

Volume of gas = mass of gas/Mr of gas x24
Eg calculate the volume of 5g of O2 at rtp
Vol = 5/32 x 24
Vol = 3.75dm3