National Chemistry

Calculations from equations

step 1: Copy out equation and underline the chemical you are asked about and the chemical you are given a
figure for.
step 2: Write underneath each of these chemicals the number of moles of each
step 3(a): Calculate the mass of 1 mole for each of these chemicals
(b): If you had more than one mole of any of these chemicals then you need to multiply the mass of 1 mole by
this value
step 4: this gives two figures to work with that you can estimate an answer from

Example 1.

Calculate the mass of water produced when 1.6g of methane is completely burned in air.

equation: CH4 + 2O2 ---> CO2 + 2H2O

Step 1: CH4  + 2O2 ---> CO2 + 2H2O
step 2: 1 mole 2 mole

step 3a: 1 mole CH4 = (1 x C) + (4 x H)
(1 x 12) + (4 x 1) = 16g

1 mole H2O = (2 x H) + (1 x O)
(2 x 1) + (1 x 16) = 18g

CH4  + 2O2 ---> CO2 + 2H2O
1 mole 2 mole
step3b: 16g 2 x 18
16g 36g

step 4: 16g of methane produces 36g of water

therefore
1g produces more than 1g but not as much as 36g

calculate  16 ----> 36
1 ----> 36
16

1.6 ---> 36  x 1.6 = 3.6g
16

example 2 Calculate the mass of ammonia produced from reacting 24g of hydrogen with excess nitrogen.

equation: N2 + 3H2 ----> 2NH3

step 1: N2 + 3H2  ----> 2NH3
step 2: 3 mole 2 mole
step 3a
1 mole of H2 = (2 x H)
= (2 x 1) = 2g

1 mole of NH3 =(1 x N) + (3 x H)
= (1 x 14) + (3 x 1) = 17g

3b: N2 + 3H2  ----> 2NH3
3 mole 2 mole
3 x 2g 2 x 17g
= 6g = 34g

step 4: 6g of hydrogen produces 34g of ammonia

therefore
1g produces more than 1g but not as much as 34g

calculate  6 ----> 34
1 ----> 34  (=5.66)
6

24 ---> 34  x 24 = 135.9g
6

Questions

2000
18. Calculate the mass of water produce when 22g of propane are completely burned in air.

C3H8 + 5O2 -----> 3CO2 + 4H2O

2001
12. Calculate the mass of sulphur produced, in grams, when 34g of hydrogen sulphide reacts with sulphur dioxide.
2H2S + SO2 -------> 3S + 2H2O

2002
Q15 Calculate the mass of iron, in tonnes, which is produced from 160 tonnes of iron(III) oxide.

Fe2O3 + 3CO -----> 2Fe + 3CO2

2003
12. Calculate the mass of zinc required to produce 0.5 mole of hydrogen.

Zn + 2HCl ----> ZnCl2 + H2
2004
nil

2005
20. Calculate the mass of hydrogen produced when 1.35g of aluminium reacts with sulphuric acid.

2Al + 3H2SO4 -----> Al2(SO4)3 + 3H2

2006
15. Calculate the mass of oxygen produced when 22g of dinitrogen monoxide decomposes

2N2O ---------> 2N2 + O2

2007
20. Calculate the mass of calcium carbonate required to neutralise 0.2 mole of hydrochloric acid

CaCO3 + 2HCl -----> CaCl2 + H2O + CO2

2008
20. Calculate the mass of iron produced from 40g of iron oxide
2Al + Fe2O3 ----> 2Fe + Al2O3

2009
19. Calculate the mass of water produced when 6.4g of nonane is burned.

C9H20 + 14O2 -----> 9CO2 + 10H2O

2010
12. Calculate the mass of hydrogen produced when 4.9g of magnesium reacts with an excess of dilute sulphuric acid.

Mg + H2SO4 ------> MgSO4 + H2

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