4.4.4 Mole ratios in equations

Question 1

What do the balancing numbers in a balanced equation for a reaction tell us?

Answer 1

How many moles of each reactant are needed and how many moles of each product are made.
e.g. N2 + 3H2 —> 2NH3
1 mole 3 moles 2 moles

Question 2

How to calculate moles and mass of other substances using the mole ratios?

Answer 2

1. Convert the mass of one substance into moles.
2. Use the mole ratio to work out moles of other substance.
3. Convert moles to mass.
   e.g. calculate the mass of N2 needed to react with 1.2g of H2 in the reaction N2 + 3H2 –> 2NH3
4. 1.2g of H2 –> 0.6 moles of H2
5. 0.6/3 = 0.2 moles of N2
6. 0.2 moles of N2 –> 5.6g of N2
   1N2 : 3H2
   1 : 3
   0.2 : 0.6

Question 3

What is meant when a substance is limiting?

Answer 3

If there is not enough of a substance, it is limiting.

Question 4

What is meant when a substance is in excess?

Answer 4

If there is more than enough of a substance, it is in excess.

Question 5

In the equation:
Zn + CuO –> ZnO + Cu
when the is 0.9 moles of Zn and 1.1 moles of CuO, which reactant is limiting and which is in excess?

Answer 5

The balancing in the equation tells us that the moles should be the same, so it must be:
Zn is limiting because there is not enough of it.
CuO is in excess because there’s too much of it.
The limiting substance determines how many moles are actually used and made, so 0.9 moles of each would react to make 0.9 moles of ZnO and Cu.

Question 6

How to work out which substance is limiting and in excess when the balancing numbers are in excess?

Answer 6

Pick one reactant at random, then calculate how much of the other would be needed to react with it and then compare that to how much of the other there actually is.
e.g. 10g of Mg and 10g of HCl
Mg + 2HCl –> MgCl2 + H2
Start with Mg:
1Mg 2HCl
mass 10 10
moles 0.416 0.274
There should be 0.833 moles of HCl instead of 0.274 moles, therefore HCl is limiting and Mg is in excess.

Question 7

What is percentage yield?

Answer 7

Percentage yield is how much product is produced against the maximum possible mass.

Question 8

How do you calculate percentage yield?

Answer 8

% yield = (experimental mass/theoretical mass) x 100
The experimental mass is measured in the experiment (or given in the question).
The theoretical mass is calculated based on the mass of a reactant.