Using a balanced chemical equation, we can calculate the mass or moles of reactants and products of a chemical reaction!! 

👇 You can follow these steps to work out the mass of products or reactants of a chemical reaction 👇

1. Write down a balanced chemical equation.
2. Using given information, calculate the amount of products/reactants in moles.
3. Using molar ratio, calculate the amount of products/reactants that you are interested in.The ratio of coefficients in front of each reactant/product is the equal mole ratio. For example, in the reaction of production of ammonia (3H₂ + N₂ → 2NH₃), 3 moles of hydrogen and 1 mole of nitrogen react to form 2 moles of ammonia.
4. Convert moles of products/reactants into masses.

Let's take a look at some examples from IGCSE Chemistry 

Example 1

The equation for the reaction between lithium nitride and water is 

Li₃N(s) + 3H₂O(l) → 3LiOH(aq) + NH₃(g)

A sample of 1.40 g of lithium nitride is added to an excess of water. Calculate the mass of lithium hydroxide formed in the reaction.

Step 1. Balanced chemical equation is:

Li₃N(s) + 3H₂O(l) → 3LiOH(aq) + NH₃(g)

Step 2. Calculate moles of lithium nitride added.

M_r of Li₃N = 6.9 x 3 + 14.0 = 34.7 g/mol

moles of Li₃N = 1.40 / 34.7 = 0.0403 mol

Step 3. Calculate moles of lithium hydroxide produced.

The molar ratio of lithium nitride and lithium hydroxide is 1 : 3.

moles of lithium hydroxide = 3 x 0.0403 = 0.121 mol

Step 4. Convert moles of lithium hydroxide to mass.

M_r of LiOH = 6.9 + 16.0 + 1.0 = 23.9 g/mol

mass of LiOH = 0.121 x 23.9 = 2.89 mol

Example 1

The equation for the reaction between lithium nitride and water is 

Li₃N(s) + 3H₂O(l) → 3LiOH(aq) + NH₃(g)

A sample of 1.40 g of lithium nitride is added to an excess of water. Calculate the mass of lithium hydroxide formed in the reaction.

Step 1. Balanced chemical equation is:

Li₃N(s) + 3H₂O(l) → 3LiOH(aq) + NH₃(g)

Step 2. Calculate moles of lithium nitride added.

M_r of Li₃N = 6.9 x 3 + 14.0 = 34.7 g/mol

moles of Li₃N = 1.40 / 34.7 = 0.0403 mol

Step 3. Calculate moles of lithium hydroxide produced.

The molar ratio of lithium nitride and lithium hydroxide is 1 : 3.

moles of lithium hydroxide = 3 x 0.0403 = 0.121 mol

Step 4. Convert moles of lithium hydroxide to mass.

M_r of LiOH = 6.9 + 16.0 + 1.0 = 23.9 g/mol

mass of LiOH = 0.121 x 23.9 = 2.89 mol

Example 2

Slaked lime is often added to soil to raise the pH of the soil. A chemist neutralises 25.0 cm³ of 0.500 mol/dm³ hydrochloric acid with slaked lime. 

Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Calculate the minimum mass, in grams, of Ca(OH)₂ required to neutralise the HCl.

Step 1. The balanced chemical equation is:

Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Step 2. Calculate moles of hydrochloric acid that is neutralized.

moles of HCl = 25.0 x 0.500 / 1000 = 0.0125 mol

Step 3. Calculate moles of Ca(OH)₂ needed to neutralize hydrochloric acid.

From the balanced equation, you can see that one mole of Ca(OH)₂ reacts with two moles of HCl. 

Thus, the moles of Ca(OH)₂ needed is half of the moles of HCl.

moles of Ca(OH)₂ = 0.5 x 0.0125 = 0.00625 mol

Step 4. Convert moles of calcium hydroxide into mass.

M_r of Ca(OH)₂ = 74 g/mol

mass of Ca(OH)₂ = 0.00625 x 74 = 0.4625 g

Example 3

Submarines that spend a long time underwater use sodium peroxide (Na₂O₂) to absorb carbon dioxide (CO₂) from the air in the submarine. The equation for the reaction is

2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂ 

There are 140 people on the submarine. Each person produces 480 dm³ of carbon dioxide per day. (Assume 1 mol of CO₂ occupies 24.0 dm³). Calculate the mass, in kilograms, of sodium peroxide required to absorb all of the carbon dioxide produced in the submarine in one day.

Step 1. The balanced chemical equation is:

2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂ 

Step 2. Calculate moles of carbon dioxide produced in one day.

volume of carbon dioxide produced per day = 140 x 480 = 67200 dm³.

moles of carbon dioxide produced per day = 67200 /  24.0 = 2800 mol

Step 3. Calculate moles of sodium peroxide required.

From the chemical equation, we can see that the molar ratio of sodium peroxide and carbon dioxide is 2:2, or 1:1. That is, you need 1 mole of sodium peroxide to absorb 1 mole of carbon dioxide produced.

Therefore, 2800 moles of sodium peroxide is required to absorb carbon dioxide produced in one day.

Step 4. Convert moles of sodium peroxide into mass.

M_r of Na₂O₂ = 23.0 x 2 + 16.0 x 2 = 78.0 g/mol

mass of Na₂O₂ = 2800 x 78.0 = 218400 g = 218.4 kg

Wanna Boost Up Your IGCSE Chemistry?