A student adds an alkali gradually to an acid. The measured pH values after successive additions are 2, 4, 6, 7, 9 and 11.
.1
Describe the change and identify the neutral reading.
[2 marks]
.2
Explain neutralisation in terms of the reacting ions.
[2 marks]
6 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A sample contains very long covalent molecules. It softens when heated, does not conduct electricity and becomes solid again on cooling.
Explain these observations using a polymer model.
[6 marks]
4 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
In paper chromatography, dye A travels farther than dye B in the same time. Both start on the same line and the solvent front has passed both spots.
Explain the separation in terms of the mobile and stationary phases, and assess the suggestion that A must be a smaller molecule.
[4 marks]
5 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
Propane burns completely in oxygen to form carbon dioxide and water. A sample uses 8.0 g oxygen and produces 6.6 g carbon dioxide and 3.6 g water. No products escape before weighing.
.1
Write the balanced equation for complete combustion of propane, C3H8.
[2 marks]
.2
Calculate the mass of propane that reacted.
[3 marks]
4 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A computer model displays 70 reactant particles and 30 product particles at equilibrium. During the next second 6 reactant particles change to product.
.1
Calculate the percentage of reactant particles that change to product in that second. Give your answer to at least two significant figures.
[2 marks]
.2
Predict the number changing back in the same second and explain why the display can stay at 70:30.
