Topic 3 - Quantitative Chemistry

Question 1

What does the law of conservation of mass state?

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.

Question 2

What is the relative formula mass?

Answer 2

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.

Question 3

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

Answer 3

Multiply the Ar of the element by the number of atoms of that element in the formula of the compound. Then divide by the compounds Mr and multiply by 100.

Question 4

In a balanced chemical equation, the sum of the Mr of the reactants equals what?

Answer 4

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

Compound X has Mr = 30 and contains 10% hydrogen mass. Ar of H = 1. How many hydrogen atoms are there in the molecular formula of compound X?

Answer 5

Mass of H = 10% of Mr of X = 10% of 30 = 3
So number of H atoms = 3/(Ar of H) = 3/1 = 3.

Question 6

What is the definition of a mole?

Answer 6

One mole is 6.02 x 10 (23) (the Avogrado constant) particles of a substance.

Question 7

How is the Ar or Mr of a substance related to the idea of a mole?

Answer 7

The value of Ar of an element or the Mr of a compound is equal to the mass in grams of 1 mole of the substance. E.g, the Mr of CO(2) has a mass of 44g.

Question 8

Which contains more particles, a mole of water or a mole of oxygen gas?

Answer 8

Neither - they both contain the same number of particles.

Question 9

What’s the formula for the number of moles in a given mass?

Answer 9

number of moles = mass in grams/Mr

Question 10

The formula of magnesium bromide is MgBr(2). Ar of Mg = 24, Ar of Br = 80. What mass of bromine is there in 0.25 moles of MgBr(2)?

Answer 10

1 mole of MgBr(2) contains 2 moles of Br atoms,
so mass of Br in 1 mole of MgBr(2) = 2 x 80 = 160g.
So mass of Br in 0.25 moles = 160/4 = 40g.

Question 11

What is meant by the term ‘conservation of mass’?

Answer 11

Conservation of mass means the total mass of the products in a reaction will always be equal to the total mass of the reactants. This is because no atoms are ever destroyed or created during a reaction.

Question 12

How does the balanced symbol equation for a reaction show that mass is conserved in that reaction?

Answer 12

In a balanced symbol equation, the sum of the relative masses of the reactants will equal the sum of the relative masses of the products. / There will be the same number of each type of atoms on both sides of the equation.

Question 13

Why might mass appear to increase in a reaction? (2)

Answer 13

• The particles in a gas move around and fill the space they’re in. So 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 from part of the product, the particles become contained inside the reaction vessel - so the total mass of the stuff inside the reaction vessel increases.

Question 14

Why might mass appear to decrease in a reaction? (2)

Answer 14

• Before the reaction, all the reactants are contained in the reaction vessel.
• If the vessel isn’t enclosed, then the gas that’s produced can escape from the reaction vessel as it’s formed. This can be explained by the particles model, which states that a gas will expand to fill any container its in. So if a reaction vessel isn’t sealed, the gas expands out of the vessel and escapes into the air around, so its mass can no longer be accounted for - appears to have decreased.

Question 15

The reaction between sodium and water is ; 2Na + 2H(2)O => 2NaOH + H(2). What is the ratio of the total number of moles of reactants to the total number of moles of products in this reaction?

Answer 15

4:3 . Every 2 + 2 = 4 moles of reactants makes 2 + 1 = 3 moles of products.

Question 16

Zinc reacts with oxygen to form zinc oxide, ZnO, as follows: 2Zn + O(2) => 2ZnO. Find the total number of moles of reactants needed to make 8 moles of ZnO.

Answer 16

The equation makes 2 moles of ZnO, so to make 8 moles of ZnO, multiply by 8/2 = 4:
8Zn + 4O(2) => 8ZnO, so making 8 moles of ZnO uses 8 + 4 = 12 moles of reactants.

Question 17

0.9 moles of compound A react completely with 0.6 moles of compound B to form 0.6 moles of compound C, which is the only product of this reaction.
Find the balanced equation for this reaction in terms of A, B and C.

Answer 17

Divide all the numbers of moles by the smallest number of moles (0.6):
A: 0.9 ÷ 0.6 = 1.5
B: 0.6 ÷ 0.6 = 1
C: 0.6 ÷ 0.6 = 1
Now multiply to get them all to be whole numbers. So in this case, multiply by 2:
A: 1.5 × 2 = 3
B: 1x2 = 2
C: 1 x 2 = 2.
So the balanced equation is:
3A + 2B → 2C

Question 18

Compound Z decomposes to make nitrogen gas, N(2), and sodium, Na.
When 2 moles of Z decomposes, it produces 46 g of sodium and 84 g of nitrogen.
Deduce the balanced equation for the decomposition of Z, in terms of Z, N(2) and Na.
Ar of Na = 23, Ar of N = 14.

Answer 18

46 g of Na = 46 ÷ 23 = 2 moles
84 g of N2 = 84 = (2 x 14) = 84 ÷ 28 = 3 moles
The numbers of moles are all whole numbers, so the balanced equation is:
2Z → 2Na + 3N(2)
(In case you were wondering, the mysterious compound Z is in fact sodium azide, NaN.)

Question 19

What’s the definition of a limiting reactant?

Answer 19

A limiting reactant is the reactant that gets completely used up in a given reaction.

Question 20

What does it mean if a reactant is said to be in excess/When is a reactant in excess?

Answer 20

A reactant is in excess if there is more than enough of it present to allow the limiting reactant to be fully used up.

Question 21

Magnesium reacts with hydrochloric acid as follows: Mg + 2HCl → MgCl + H2
How many moles of magnesium chloride, MgCl, form when 0.4 moles of magnesium react with an excess of hydrochloric acid?

Answer 21

0.4 moles. The equation shows you get 1 mole of MgCl(2), for each mole of Mg that reacts. The acid is in excess, so all 0.4 moles of magnesium will react to make 0.4 moles of MgCl(2).

Question 22

Methane, CH(4), burns according to the following equation:
CH(4) + 20(2) → CO(2) + 2H(2)O.
What mass of water is formed when 64 g of methane burns in air?
Mr of CH(4)= 16, Mr of H(2)0 = 18.

Answer 22

Number of moles in 64 g of CH(4) = mass/Mr = 64/16 = 4
From the equation, each mole of CH(4) makes 2 moles of H(2)O, so 2 x 4 = 8 moles of H(2)0 are made.
So mass of H(2)O produced = moles x Mr = 8 x 18 = 144 g.

Question 23

Fluorine reacts with water to form hydrogen fluoride, HF(2) The reaction equation is
2F(2) + 2H(2)O → O(2) + 4HF
When 76 g of F, reacts with an excess of water, what mass of hydrogen fluoride is produced?
Ar of F = 19, Ar of H = 1.

Answer 23

Mr of F(2)= 2 x 19 = 38, so moles of F(2) reacting = mass/Mr= 76 ÷ 38 = 2.
From the equation, 2 moles of F(2) produces 4 moles of HF.
Mr of HF = 1 + 19 = 20, so mass of HF produced = moles x M = 4 x 20 = 80 g.

Question 24

What’s the volume of one mole of any gas at room temperature and pressure?

Answer 24

24 dm(3). The same number of moles of any two gases will always occupy the same volume as each other if they’re at the same temperature and pressure.

Question 25

What’s the formula for calculating the volume of a gas at r.t.p. from its mass?

Answer 25

volume of gas = mass of gas/Mr of gas x 24

Question 26

What volume does 1.6 kg of bromine gas, Br(2), occupy at r.t.p.? Ar of Br = 80.

Answer 26

The mass needs to be in grams. 1.6 kg = 1.6 x 1000 = 1600 g.
Mr of Br(2) = 2 × 80 = 160.
So volume = (mass of gas/Mr of gas) × 24 = (1600 ÷ 160) × 24 = 10 × 24 = 240 dm(3).

Question 27

The equation for the decomposition of calcium carbonate is:
CaCO(3) → CaO + CO(2)
What is the volume at r.t.p. of the carbon dioxide produced when 25 g of calcium carbonate fully decomposes?
Ar of Ca = 40, Ar of C = 12, Ar of 0 = 16.

Answer 27

Moles of CaCO(3) = mass/Mr = 25/(40 + 12 + (3 x 16)) = 25 ÷ 100 = 0.25
From the equation, 0.25 moles of CaCO(3) produces 0.25 moles CO(2).
Volume at r.t.p. of 0.25 moles of CO(2) = 0.25 x 24 = 6 dm(3).

Question 28

How do you calculate concentration in g/dm(3)?

Answer 28

Concentration = mass in g / volume in dm(3)

Question 29

How do you calculate concentration in mol/dm(3)?

Answer 29

Concentration = number of moles / volume in dm(3)

Question 30

How do you convert concentration from mol/dm(3) to g/dm(3)?

Answer 30

Concentration in g/dm(3) = concentration in mol/dm(3) x Mr

Question 31

What is the best method for calculating concentrations etc, particularly from an equation?

Answer 31

The grid method, including;
* Molar ratio
* Number of Moles
* Concentration
* Volume

Question 32

What is atom economy?

Answer 32

The atom economy of a reaction is the percentage of the mass of the reactants that ends up as part of the desired product.

Question 33

What does it tell you if a reaction has an atom economy of 100%?

Answer 33

All the atoms in the reactants become part of the desired products, so no waste is produced.

Question 34

What’s the formula for calculating atom economy?

Answer 34

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

Question 35

The follow reaction can be used to produce calcium oxide;
CaCO(3) => CaO + CO(2)
Find the atom economy of the reaction.
Ar of Ca = 40
Ar of C = 12
Ar of O = 16.

Answer 35

Atom economy = [Mr of CaO / Mr of CaCO(3)] x 100 = [(40 + 16) / (40 + 12 + (16 x 3 ))] x 100 = [56 / 100] x 100 = 56%.

Question 36

What are 3 advantages of using a reaction with a high atom economy in industry?

Answer 36

• High atom economy reactions use less raw materials, so are more sustainable than low atom economy reactions.
• There is less waste to be disposed of with high atom economy reactions than low atom economy reactions.
• High atom economy reactions tend to be more profitable than low atom economy reactions, as costs of buying materials and disposing of waste are lower.

Question 37

What is the formula for working out the percentage yield of a reaction?

Answer 37

percentage yield = mass of product actually made / maximum theoretical mass of product x 100

Question 38

Why can you never get 100% yield from a reversible reaction?

Answer 38

In a reversible reaction, both the forward and backwards reactions happen at the same time, so some of the products always get converted back to the reactants and the reaction never goes to completion.

Question 39

Marlon carried out a reaction in the lab. He calculated his maximum yield would be 3.2 g of product. He measured the actual yield as 3.5 g. How can you tell Marlon has made a mistake? Suggest three possible mistakes that could have led to this error.

Answer 39

• Marlon’s actual yield is greater than his maximum theoretical yield, so his percentage yield is greater than 100%. This is impossible, so he must have made a mistake.
• E.g. he could have calculated his maximum yield incorrectly, measured out more of his reactants than he intended, or failed to separate out some impurities his product.

Question 40

When choosing which reaction to use to make a substance on an industrial scale, which 5 factors need to be considered?

Answer 40

• Atom economy - ideally high
• Yield - ideally high
• Rate - reasonably fast
• Equilibrium position - ideally the amount of products formed is high and amount of reactants left is low
• Ideally, byproducts produced are useful