The Mole and Equations (pg 126)

Question 1

The big numbers in front of the chemical formulas of the reactants and products tell you how many moles of each substance takes part or is formed during the reaction.

What do this equation mean?

Mg(s) + 2HCl(aq) &raquo_space;> MgCl2(aq) + H2(g)

Answer 1

In this reaction, 1 mole of magnesium and 2 moles of hydrochloric acid react together to form 1 mole of magnesium chloride and 1 mole of hydrogen gas

(the little numbers tell you how many atoms of each element there are in each of the substances)

Question 2

How do you balance equations using reacting masses?

Answer 2

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. Here are the steps you should take:

1. Divide the mass of each substance by it’s relative formula mass to find the number of moles.
2. Divide the number of moles of each substance by the smallest number of moles in the reaction.
3. If any of the number aren’t whold numbers, multiply all the umbers by the same amount oso 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 3

8.1g of zinc oxide (ZnO) reacts completely with 0.60g of carbon to form 2.2g of carbon dioxide and 6.5g of zinc. Write a balanced symbol equation for this reaction?

Ar(C) = 12, Ar(O) = 16, Ar(Zn) = 65

Answer 3

1. Work out Mr for each of the substances in the reaction:

ZnO: 65 + 16 = 81 C: 12 CO2: 12 + (2 x 16) = 44 Zn: 65

2. Divide the mass of each substance by it Mr to calculate how many moles of each substance reacted or were produced:

ZnO: 8.1 C: 0.60
__ = 0.10 mol ___ = 0.050 mol
81 12

CO2: 2.2 Zn: 6.5
___ = 0.050 mol ___ = 0.10
44 65

3. Divide by the smallest number of moles, which is 0.050:

ZnO: 0.10 C: 0.050
____ = 2.0 _____ = 1.0
0.050 0.050

CO2: 0.050 Zn: 0.10
_____ = 1.0 _____ = 2.0
0.050 0.050

(these numbers give the ratio of the amounts of each substance in the reaction equation).

4. the numbers are all whole numbers, so you can write out the balanced symbol equation straight away.

2ZnO + C&raquo_space;> CO2 + 2Zn

Question 3

8.1g of zinc oxide (ZnO) reacts completely with 0.60g of carbon to form 2.2g of carbon dioxide and 6.5g of zinc. Write a balanced symbol equation for this reaction?

Ar(C) = 12, Ar(O) = 16, Ar(Zn) = 65

Answer 3

1. Work out Mr for each of the substances in the reaction:

ZnO: 65 + 16 = 81 C: 12 CO2: 12 + (2 x 16) = 44 Zn: 65

2. Divide the mass of each substance by it Mr to calculate how many moles of each substance reacted or were produced:

ZnO: 8.1 C: 0.60
__ = 0.10 mol ___ = 0.050 mol
81 12

CO2: 2.2 Zn: 6.5
___ = 0.050 mol ___ = 0.10
44 65

3. Divide by the smallest number of moles, which is 0.050:

ZnO: 0.10 C: 0.050
____ = 2.0 _____ = 1.0
0.050 0.050

CO2: 0.050 Zn: 0.10
_____ = 1.0 _____ = 2.0
0.050 0.050

(these numbers give the ratio of the amounts of each substance in the reaction equation).

4. the numbers are all whole numbers, so you can write out the balanced symbol equation straight away.

2ZnO + C&raquo_space;> CO2 + 2Zn

Question 4

84g of N2 reacts completely with 18g of H2 to produce 102g of NH3.
Mr(N2) = 28, Mr(H2)=2, Mr(NH3)=17

a) calculate how any moles of each substance reacted or was produced (3 marks)

b) Use your answer to part a) to write a balanced symbol equation for this reaction (2 marks)

Answer 4

Please scan QR code on page 126 for video of working this out)

a) N2: 84
__ = 3.0 mol (1 mark)
28

  H2:  18
           \_\_   =  9 mol  (1 mark)
            2

  NH3:  102
             \_\_\_  = 6.0 mol (1 mark)
               17

b) Divide by the smallest number of moles (3.0)

N2: 3.0 H2: 9 NH3 6.0
___ = 1 __ = 3 ___ = 2.0 (1 mark)
3.0 3.0 3.0

The balanced symbol equation is:

N2 + 3H2 &raquo_space;» 2NH3 (1 mark)