Question 1

What's the difference between RTP and STP?

Accepted Answer

RTP (Room Temperature and Pressure) is typically 298 K (25 °C) and 1 atm, with a molar volume of 24 dm³ mol⁻¹. STP (Standard Temperature and Pressure) is 273 K (0 °C) and 1 atm, with a molar volume of 22.4 dm³ mol⁻¹. For A-Level Chemistry (9701), you will almost always use RTP and the value of 24 dm³ mol⁻¹ will be given in the exam data.

Question 2

How do I know which reactant is limiting if the question doesn't state it?

Accepted Answer

If you are given the starting amounts (mass, volume, etc.) of more than one reactant, you MUST check for a limiting reactant. Calculate the moles of each, then compare their ratio to the ratio in the balanced equation. A systematic way is to divide the moles of each reactant by its coefficient in the equation; the smallest result identifies the limiting reactant.

Question 3

Why do we have to convert volume to dm³ for concentration calculations?

Accepted Answer

The standard unit for concentration is moles per cubic decimetre (mol dm⁻³). To ensure the units are consistent and cancel correctly in the formula $n = c 	imes V$, the volume must be in dm³. Using cm³ directly with a concentration in mol dm⁻³ will give an answer that is 1000 times too small.

Question 4

Do I lose marks for not using the correct number of significant figures?

Accepted Answer

Yes. As a general rule, your final answer should be given to the same number of significant figures as the least precise piece of data used in the calculation. For example, if you are given a mass of 2.51 g (3 s.f.) and a concentration of 0.10 mol dm⁻³ (2 s.f.), your final answer should be given to 2 significant figures. It is good practice to keep extra figures during intermediate steps and only round at the very end.

Reacting masses and volumes (of solutions and gases)

What you need to know

The Chemist's Recipe Book