Quantitative chemistry
Chemistry · GCSE · The School
Counting by weighing
You cannot count atoms, so chemists weigh them. RELATIVE FORMULA MASS (Mr) is the sum of the atomic masses in a formula: water, H2O, is 1 + 1 + 16 = 18. A MOLE is simply a fixed number of particles, chosen so that one mole of a substance weighs its Mr in grams. Moles = mass ÷ Mr, and almost every calculation in this topic is that one line rearranged.
The equation is the recipe
A balanced equation gives the RATIO. In 2H2 + O2 → 2H2O, two moles of hydrogen react with one of oxygen. Work in moles, never in grams: convert mass to moles, use the ratio from the equation, convert back to mass. Trying to use the ratio on grams directly is the commonest error in the topic and it is always wrong.
Why you never get it all
PERCENTAGE YIELD = (actual ÷ theoretical) × 100. It is below 100 because product is lost when transferred, the reaction may not go to completion, and side reactions make something else. ATOM ECONOMY is different: it asks what proportion of the atoms you started with end up in the product you wanted, and a reaction can have a high yield and a poor atom economy.
Mg + O₂ → MgO is not balanced: two oxygen atoms on the left, one on the right. Fix it: 2Mg + O₂ → 2MgO. Now use it. Relative masses: Mg = 24, O = 16, so MgO = 40. Burn 4.8 g of magnesium: that is 4.8 ÷ 24 = 0.2 mol Mg, which gives 0.2 mol MgO, which is 0.2 × 40 = 8.0 g. Balance first, always — the unbalanced equation would have predicted the wrong mass.
Concentration, and the units that trip people
Concentration can be given in grams per cubic decimetre or in moles per cubic decimetre, and converting between them needs the relative formula mass. A cubic decimetre is a litre, which is 1000 cm³ — most arithmetic errors here are the factor of a thousand, not the chemistry.
Limiting reactant decides the yield
When two reactants are given, one runs out first and caps the product; the other is in excess. Work out the moles of each, divide by the coefficients in the balanced equation, and the smaller answer is limiting. Calculating yield from the excess reactant is a common and expensive mistake.