Reacting Masses (AQA A-Level Chemistry): Revision Notes

Example: Calculating the Mass of Oxygen Required to React with Hydrogen Question: What mass of oxygen gas (O₂) is needed to react with hydrogen gas (H₂) to produce 9 g of water (H₂O)?

Step 1: Write the balanced chemical equation:

Step 2: Calculate moles of water: The molar mass of water (H₂O) is 18 g/mol.

Step 3: Use the mole ratio: From the balanced equation, 2 moles of H₂O are produced by 1 mole of O₂.

Therefore, for 0.5 moles of water, the required moles of oxygen are:

Step 4: Convert moles of O₂ to mass: The molar mass of oxygen gas (O₂) is 32 g/mol.

Therefore, 8 g of oxygen is required.

Example: Mass of a Reactant or Product: Question: How much calcium carbonate (CaCO₃) is required to produce 44 g of carbon dioxide (CO₂) in the reaction:

Solution:

• $M_r$ of CO₂ = 44 g/mol
• So moles of CO₂ = 44 g / 44 g/mol = 1 mol.
• From the balanced equation, 1 mol of CaCO₃ produces 1 mol of CO₂
• So moles of CaCO₃ = 1 mol.
• $M_r$ of CaCO₃ = 100 g/mol
• So the mass of CaCO₃ required = 1 mol × 100 g/mol = 100 g.

Example: Mass of a Compound in Solution: Question: How much sodium hydroxide (NaOH) is needed to completely react with 50 cm³ of 1 mol/dm³ hydrochloric acid (HCl) in the reaction:

Solution:

• Moles of HCl = 1 mol/dm³ × (50 cm³ / 1000 cm³/dm³) = 0.05 mol.
• From the balanced equation, 1 mole of NaOH reacts with 1 mole of HCl.
• So, moles of NaOH = 0.05 mol.
• $M_r$ of NaOH = 40 g/mol.
• Therefore, mass of NaOH = 0.05 mol × 40 g/mol = 2 g.

Summary

Reacting masses calculations are essential for determining the quantities of reactants or products in a chemical reaction. The steps involve converting mass to moles, using the balanced chemical equation to find the mole ratio, and then calculating the mass or volume of the unknown species. Mastering these calculations is crucial for solving many types of chemistry problems.