Building Blocks for Understanding: Chemical Quantities — AQA Combined Science: Synergy
This topic covers chemical equations, conservation of mass, relative formula mass, moles and concentration.
Chemical equations
Reactions are represented by balanced symbol equations — the same number of each type of atom on both sides. State symbols (s, l, g, aq) show the physical states.
Conservation of mass
In a chemical reaction, no atoms are created or destroyed, so the total mass of products equals the total mass of reactants. Mass can appear to change if a gas is lost (mass decreases) or a gas is gained from the air (mass increases); in a sealed container mass stays constant.
Relative formula mass (Mr)
The relative formula mass (Mr) is the sum of the relative atomic masses (Ar) of all the atoms in a formula.
Example — water, H₂O: Mr = (2 × 1) + 16 = 18.
In a balanced equation, the total Mr of the reactants equals the total Mr of the products.
Amounts in moles (Higher Tier)
Chemical amounts are measured in moles (mol). One mole contains 6.02 × 10²³ particles (the Avogadro constant), and its mass in grams equals the relative formula mass.
$$\text{moles} = \frac{\text{mass (g)}}{\text{Mr}}$$
Calculations based on equations (Higher Tier)
The balanced equation gives the mole ratio of reactants and products. To find a reacting mass:
- Balance the equation.
- Convert the known mass to moles.
- Use the ratio to find moles of the wanted substance.
- Convert back to mass.
The reactant used up first is the limiting reactant; it determines the amount of product. The other is in excess.
Concentrations of solutions
$$\text{concentration (g/dm}^3) = \frac{\text{mass of solute (g)}}{\text{volume (dm}^3)}$$
Remember 1 dm³ = 1000 cm³. More solute in a given volume (or a smaller volume for the same mass) gives a more concentrated solution.
Worked example: dissolving 20 g in 250 cm³ → 20 ÷ 0.25 = 80 g/dm³.
Exam tips
- Always balance the equation first in calculations.
- Watch units — convert cm³ to dm³ by dividing by 1000.
- Learn Mr, moles = mass ÷ Mr and concentration = mass ÷ volume.
- Identify when mass appears to change because a gas is lost or gained.