C3.2 Introduction to quantitative chemistry · KS3 Science

Relative formula mass

The relative atomic mass (Ar) of each element is on the periodic table: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Cl = 35.5, Ca = 40.
The relative formula mass (Mr) of a compound is the sum of the Ar values of all the atoms in its formula.
H₂O: (2 × 1) + 16 = 18. CO₂: 12 + (2 × 16) = 44. CaCO₃: 40 + 12 + (3 × 16) = 100.

Percentage by mass of an element = (Ar × number of atoms of that element) ÷ Mr × 100.
Calcium in calcium carbonate (Mr 100): 40 ÷ 100 × 100 = 40%.
Oxygen in magnesium oxide (Mr 40): 16 ÷ 40 × 100 = 40%.

In a balanced equation, the total Mr of the reactants equals the total Mr of the products, because atoms are only rearranged.
2H₂ + O₂ → 2H₂O: 4 + 32 = 36, and 2 × 18 = 36.
Mg + 2HCl → MgCl₂ + H₂ is balanced: 1 magnesium, 2 hydrogen and 2 chlorine atoms on each side.

Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³). 1 dm³ = 1000 cm³, so divide volumes in cm³ by 1000.
10 g of salt in 0.5 dm³ of solution has a concentration of 10 ÷ 0.5 = 20 g/dm³.
Repeat readings show how uncertain a result is: uncertainty = ± half the range. Readings of 20.1 s, 20.5 s and 20.3 s have a mean of 20.3 s and an uncertainty of ±0.2 s.
Soluble salts are made by adding an excess of an insoluble metal oxide or carbonate to an acid, filtering off the extra solid, then crystallising the solution.
All sodium, potassium and ammonium salts, and all nitrates, are soluble. Most chlorides are soluble, but silver chloride isn't. Most carbonates are insoluble.

Relative atomic mass: The mass of an atom compared with an atom of carbon-12 (Ar).
Relative formula mass: The sum of the relative atomic masses of all the atoms in a formula (Mr).
Percentage by mass: The percentage of a compound's mass that comes from one element.
Conservation of mass: The total mass of the products equals the total mass of the reactants.
Balanced equation: A symbol equation with the same number of each atom on both sides.
Concentration: The mass of solute in a certain volume of solution.
Solute: The substance that dissolves.
Uncertainty: How much a measured value could vary: plus or minus half the range.
Soluble salt: A salt that dissolves in water.
Excess: More than is needed to react completely.