SACE Chemistry calculations: titrations, Kc and enthalpy

Most marks for calculations come from the steps: a balanced equation, moles from the data, the mole ratio, then the quantity asked for, to sensible significant figures with a unit.

Data sheet	n = m/M, c = n/V, Q = mcΔT, ΔH = Q/n, pH = −log[H⁺]
Not given	Constants such as Kw and the specific heat of water are stated in the question
Working	Show each step, with units and significant figures

The calculations to know

Concentration in mol L⁻¹, g L⁻¹, % w/v, ppm and ppb, and converting between them.
Titrations: moles of the standard, the mole ratio, then scaling an aliquot back to the whole sample.
pH from [H⁺] or [OH⁻], using Kw at 25 °C.

Try 2 questions

Question 1

Titres of sodium hydroxide solution: rough 24.90 mL; trial 1 24.15 mL; trial 2 24.20 mL; trial 3 24.45 mL; trial 4 24.15 mL.
Titration	Rough	1	2	3	4
Titre (mL)	24.90	24.15	24.20	24.45	24.15

A student's titration results are shown in the table. Titres within 0.10 mL of each other are concordant. What is the mean concordant titre?

Question 2

Molar enthalpy of combustion, molar mass and density of ethanol and octane.
Fuel	ΔcH (kJ mol⁻¹)	M (g mol⁻¹)	Density (g mL⁻¹)
ethanol	−1367	46.07	0.789
octane	−5470	114.22	0.703

A fleet manager compares ethanol with octane, the main component of petrol, as fuels, using the data in the table. How many times as much heat does one litre of octane release as one litre of ethanol, when each burns completely?

Next step

Take the free test Open the titrations lesson

Questions people ask

How many significant figures should I give?

Usually as many as the least precise value in the question, often three. A pH is usually given to two decimal places.