Counting particles by mass: the mole: notes and practice questions
- This topic covers no additional higher level material beyond the standard level content.
- Higher level students are expected to master the standard level concepts of the mole, relative atomic and formula mass, and molar mass.
- Key skills include interconverting mass, moles, and number of particles, and determining empirical and molecular formulas.
- Calculations involving molar concentration and gas volumes based on Avogadro's law are also essential.
How it is examined
Everywhere. This is the highest-traffic subtopic in the course. May 2025 HL Paper 2 TZ1 4(a)(ii) asked for n(CaCO₃) and n(HCl) then which reactant was limiting, [2], with a Notes instruction not to award the second mark unless working or an answer was seen for the first. 4(a)(iii) then used the molar volume from the booklet for [1]. May 2025 HL Paper 1B TZ1 1(a) asked for the molar mass of Na₂S₂O₃ and then the mass needed for a given volume and concentration, [2], with [2] for a correct final answer. Chained mole calculations are where error carried forward matters most: a wrong Mr followed by correct method still earns the later marks.
The Avogadro constant NA. Relative atomic masses to two decimal places in the periodic table. The relationships n = m / M and n = CV. Molar volume of an ideal gas at STP. What is recall: how to build an empirical formula from percentage composition, and how to scale it to a molecular formula.
- 1.4.1 The mole (mol) is the SI unit of amount of substance. One mole contains exactly the number of elementary entities given by the Avogadro constant. Students convert the amount of substance n to the number of specified elementary entities.
- 1.4.2 Masses of atoms are compared on a scale relative to ¹²C and expressed as relative atomic mass Ar and relative formula mass Mr. Students determine Mr from Ar values.
- 1.4.3 Molar mass M has units g mol⁻¹. Students solve problems involving the relationships between number of particles, amount of substance in moles, and mass in grams, using n = m / M.
- 1.4.4 The empirical formula gives the simplest ratio of atoms of each element; the molecular formula gives the actual number of atoms of each element in a molecule. Students interconvert percentage composition by mass and empirical formula, and determine molecular formula from empirical formula and molar mass.
Guiding questions
- How do we quantify matter on the atomic scale?
Linking questions
- Reactivity 2.1 How can molar masses be used with chemical equations to determine the masses of the products of a reaction?
- Tool 1 How can experimental data on mass changes in combustion reactions be used to derive empirical formulas? What are the considerations in the choice of glassware used in preparing a standard solution and a serial dilution?
- Tool 1, Inquiry 2 How can a calibration curve be used to determine the concentration of a solution?
- Nature of science, Tool 3, Structure 3.2 What is the importance of approximation in the determination of an empirical formula?
- Structure 1.5 Avogadro's law applies to ideal gases. Under what conditions might the behaviour of a real gas deviate most from an ideal gas?
Practice questions
2 questions · 1 easy · 1 mediumQuestion 1
EasyPaper 1A · calculator1 markDetermine the total number of oxygen atoms present in mol of glucose, .
A.
B.
C.
D.
E.
Recall the definition of a mole and Avogadro's constant. Pay close attention to the chemical formula to find the number of oxygen atoms per molecule.
Question 2
MediumPaper 1A · calculator1 markof () is dissolved in water to prepare of aqueous solution.
What is the concentration of copper(II) ions, in , in the resulting solution?
A.
B.
C.
D.
First, calculate the moles of using its given mass and molar mass. Then, consider the stoichiometry of the dissociation of to determine the moles of ions. Finally, use the volume of the solution to find the concentration of ions.
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Where marks are lost
- Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
- Joining the dots instead of drawing a curve.
- Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.
