Volumes of Gases (HT)AQA GCSE Chemistry: Revision notes

Many reactions happen in solution, and at Higher Tier you must be able to work in moles per dm³ as well as grams per dm³.

• Concentration (mol/dm³) = moles of solute ÷ volume of solution (dm³)
• Rearranged: moles = concentration × volume
• To find the mass of solute in a given volume of a solution of known concentration, first find the moles, then use mass = moles × Mr
• Remember to convert cm³ to dm³ by dividing by 1000 before using the formula

In a titration-style calculation you may be given the concentration and volume of one solution, plus the volume of a second solution that reacts completely with it, and asked to find the second solution's concentration.

Method:

1. Use the balanced equation to find the mole ratio between the two reactants
2. Calculate moles of the solution whose concentration you know (moles = concentration × volume)
3. Use the mole ratio to find moles of the second solution
4. Calculate its concentration: concentration = moles ÷ volume

At the same temperature and pressure, the particles in any gas are spread so far apart that the actual size of the particles barely matters — what matters is how many particles (moles) are present.

• Equal amounts, in moles, of any gas occupy the same volume under the same conditions of temperature and pressure
• At room temperature and pressure (RTP) — defined as 20 °C and 1 atmosphere — the volume of one mole of any gas is 24 dm³
• This applies equally to elements and compounds, and to mixtures of gases

To find the volume of a gas at RTP from a given mass:

1. Calculate moles = mass ÷ Mr
2. Calculate volume = moles × 24 dm³

To find volumes of gaseous reactants or products in a reaction, use the balanced symbol equation:

1. Convert the given volume of one gas into moles (moles = volume ÷ 24)
2. Use the mole ratio from the balanced equation to find moles of the gas you want
3. Convert back to volume (volume = moles × 24)

In many reactions the reactants are not present in the exact mole ratio needed by the equation. The reactant that is completely used up first is the limiting reactant — it controls the maximum amount of product that can form, however much of the other (excess) reactant is present.

• Work out the moles (or mass) of each reactant available
• Compare with the mole ratio in the balanced equation to identify which one runs out first
• Base all further calculations (mass of product, or volume of gas produced) on the moles of the limiting reactant only