WASSCE Chemistry · Paper 1 style

Laws

One sample question with the full working. 2 more in the app, with instant feedback and repair sessions for the traps below.

When 10 g of CaCO₃ decomposes completely, 5.6 g of CaO forms. The mass of CO₂ given off is

A15.6 g
B5.6 g
C4.4 g
D10 g

Show the answer and working

Answer: C

4.4 g.

Trap A Confuses laws of chemical combination: Conservation of mass: mass unchanged in a reaction. Definite proportions: a compound always has the same composition. Multiple proportions: masses of one element combining with a fixed mass of another are in simple ratios (CO, CO₂). Gay-Lussac: gas volumes in simple ratios.

Trap B Ignores the given amount and uses one mole: Convert the given quantity to moles first (n = m ÷ M or V ÷ 22.4), then apply the mole ratio from the equation, then convert back.

Trap D Takes one factor as the whole answer: Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.

Traps in Laws

Confuses laws of chemical combination
Conservation of mass: mass unchanged in a reaction. Definite proportions: a compound always has the same composition. Multiple proportions: masses of one element combining with a fixed mass of another are in simple ratios (CO, CO₂). Gay-Lussac: gas volumes in simple ratios.
Ignores the given amount and uses one mole
Convert the given quantity to moles first (n = m ÷ M or V ÷ 22.4), then apply the mole ratio from the equation, then convert back.
Takes one factor as the whole answer
Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.
Mixes up a pair of similar chemical terms
Keep a list of confusable pairs with one defining feature each and a hook. Underline the key word in the stem and check it against the definition, not the sound of the name.
Gas-law errors
Boyle: P₁V₁ = P₂V₂ (constant T). Charles: V₁/T₁ = V₂/T₂ (T in kelvin = °C + 273). General: P₁V₁/T₁ = P₂V₂/T₂.

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