CHEM - Calculating Reacting Masses
Chemistry, reacting mass - IGCSE | IBDP | DSE | GCE | IAL | AP Chemistry
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 👇
Let's take a look at some examples from IGCSE Chemistry
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:
Step 2. Calculate moles of lithium nitride added.
Mr 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.
Mr of LiOH = 6.9 + 16.0 + 1.0 = 23.9 g/mol
mass of LiOH = 0.121 x 23.9 = 2.89 mol
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:
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.
Mr of Ca(OH)2 = 74 g/mol
mass of Ca(OH)2 = 0.00625 x 74 = 0.4625 g
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 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.
Mr 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
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