A sodium hydroxide solution is added gradually to hydrochloric acid. Initially the acid has pH 1; later the mixture has pH 4.
.1
Calculate the initial hydrogen ion concentration divided by the later hydrogen ion concentration.
[2 marks]
.2
Explain the chemical change that removes hydrogen ions. Explain why pH 4 does not show complete neutralisation.
[2 marks]
4 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
A solid decomposes reversibly into two gases. In experiment A it is heated in an open tube; in experiment B it is heated in a sealed vessel. In this model, gases escape from A but remain in B.
Explain why B can establish dynamic equilibrium but A cannot establish that equilibrium while gas escapes.
[4 marks]
5 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
Two colourless liquids form a mixture. Liquid A boils at 56 °C and liquid B at 100 °C. A student wants to collect the liquid richer in A.
.1
Explain how fractional distillation separates the liquids.
[3 marks]
.2
Explain why one collected fraction is not guaranteed to be perfectly pure.
[2 marks]
6 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
Two separate samples each have mass 8.0 g. One is oxygen gas, O2, and the other is sulfur dioxide, SO2.Ar: O = 16, S = 32.
.1
Calculate the amount of oxygen molecules in the oxygen sample.
[2 marks]
.2
Calculate the amount of sulfur dioxide molecules in its sample.
[2 marks]
.3
Explain which sample contains more molecules and by what factor.
[2 marks]
5 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
An oxygen molecule contains two oxygen atoms. Each atom has six outer electrons.
.1
Explain the bonding needed for both atoms to achieve full outer shells.
[3 marks]
.2
Explain why a line drawing O=O does not show every electron.
