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1.4.3—Molar mass (M)

Syllabus
First assessment 2025
Objective
1.4.3
Level
SL

Molar Mass Conversions

Molar mass is the mass of one mole of a substance, measured in g mol⁻¹. It connects mass, amount, and particle number in a conversion chain.

n=m/Mandm=nMn = m/M and m = nM

To reach particles from mass, divide mass by molar mass to obtain moles, then use the Avogadro conversion and count the requested entities if the formula contains more than one.

Use units to choose the direction: 9.0 g of H₂O divided by 18.0 g mol⁻¹ gives 0.50 mol. To find molecules, continue from moles to nNₐ; do not multiply mass directly by the Avogadro constant.

Solving Molar-Mass Problems

Assessment in practice

1–2 marks in the selected examples marks
How it is assessed

Questions determine molar mass from mass and amount or determine the number of specified atoms from a sample mass.

Command terms

determine

What earns marks

Use n=m/M or m=nM with g mol⁻¹, then multiply by the Avogadro constant and the number of requested atoms per molecule when required.

Watch for

Using mass divided by moles in the wrong direction, or stopping at moles when the question asks for atoms.

Representative question

Question 1

[Maximum number: 2]

Calculate the number of hydrogen atoms in 1.00 g of propan-2-ol.

The Mole Summary

Retrieve the quantitative chain: count entities with mN_A, sum relative masses from formulae, convert mass with n=m/M, derive formula ratios, use n=VC for solutions, and apply gas-volume ratios at the same temperature and pressure.

Before finalising, check the requested entity, formula subscripts, units, dm³ conversion, balanced-equation coefficients, and any limiting reactant.