Edexcel IGCSE Chemistry · Spec 1.34C-1.35C
Mole Calculations: Solutions & Gas Volumes
Mole calculations involving the concentrations of solutions and volumes of gases.
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Mole Calculations: Solutions & Gas Volumes
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Explained
Moles in solutions and in gases
Two equations cover this whole topic. Concentration in moles per cubic decimetre equals moles divided by volume in cubic decimetres. Volume of a gas in cubic decimetres equals moles multiplied by 24, at room temperature and pressure. Almost every question is one of those two, wrapped in a unit conversion.
The conversion that decides the answer
Volumes in chemistry arrive in cubic centimetres and the equations need cubic decimetres. There are 1,000 cubic centimetres in one cubic decimetre, so divide by 1,000.
25 cm3 becomes 0.025 dm3. 250 cm3 becomes 0.25 dm3. Do this conversion as a separate written step before touching the equation. An answer out by a factor of 1,000 is almost always a conversion done in the head and done wrong.
Concentration
Concentration tells you how crowded a solution is: how many moles of solute sit in each cubic decimetre of solution.
Rearranged, moles equals concentration multiplied by volume in dm3. That form is the one used most often, because titration questions give you a concentration and a volume and expect the number of moles.
Concentration can also be given in grams per cubic decimetre. To convert, multiply the concentration in mol/dm3 by the relative formula mass, since one mole has a mass equal to the relative formula mass in grams.
Gas volumes
One mole of any gas occupies 24 dm3 at room temperature and pressure, which is 24,000 cm3. The identity of the gas does not matter, which is the surprising part and the part worth trusting.
So volume in dm3 equals moles multiplied by 24, and moles equals volume divided by 24. If a question gives a volume in cm3, divide by 24,000 instead, or convert first.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry describe a common pattern: many learners correctly calculated the number of moles to gain the first mark, but could not progress to the second.
The lesson is that finding moles is rarely the whole question. It is usually step one of three, and the marks beyond it come from using the ratio in the balanced equation and then converting back to a mass or a volume.
Reports also credit a candidate specifically for converting the volume into cubic decimetres in the first step before calculating moles, which is the habit worth copying.
A reliable order of working
Convert every volume to cubic decimetres. Find moles of the substance you know. Use the ratio from the balanced equation to find moles of the substance you want. Convert that into whatever the question asked for, whether a mass, a volume or a concentration.
Write each step on its own line. Method marks are available even when the final number is wrong, but only if the examiner can see the steps.
Spec 1.34C-1.35C
What you need to know
- Calculate a concentration in moles per cubic decimetre
- Convert between cubic centimetres and cubic decimetres
- Use the molar gas volume
Active recall
Quick check
Answer each question before opening the answer.
How do you calculate moles from concentration and volume?
Moles = concentration (mol/dm³) × volume (dm³).
What volume does one mole of any gas occupy at room temperature and pressure?
24 dm³ (24 000 cm³).
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