Moles Are an Amount, Not a Mass

What students typically write

“The moles of sodium is 23 g.” Or: “One mole of water weighs 18 moles.” Students use “moles” and “grams” interchangeably, treating the mole as just another word for mass.

Why it is wrong

The mole is a counting unit, like “dozen” but enormously larger. One mole = 6.02 x 10^23 particles. It tells you how many atoms, molecules, or formula units you have, not how heavy they are.

Mass (in grams) and amount (in moles) are connected by the relative formula mass (Mr) through the relationship n = m / Mr. One mole of water (Mr = 18) has a mass of 18 g. One mole of sodium (Ar = 23) has a mass of 23 g. The masses are different, but both represent the same amount: one mole, or 6.02 x 10^23 particles.

Confusing these two quantities leads to calculation errors. If a student thinks “0.5 moles of NaOH” means “0.5 grams of NaOH”, every subsequent step in a reacting mass calculation will be wrong.

What the mark scheme actually wants

A typical question: “Calculate the number of moles in 8.0 g of sodium hydroxide (NaOH). [Mr of NaOH = 40]” [2]

Marking points:

1. n = m / Mr stated or implied (1 mark)
2. n = 8.0 / 40 = 0.20 mol (1 mark)

The answer is 0.20 mol, not 0.20 g. Writing the wrong unit in the final answer can lose the mark even if the arithmetic is correct.

Worked example: wrong vs right

Question: Calculate the mass of 0.25 mol of calcium carbonate (CaCO3). [Mr of CaCO3 = 100] [2]

Wrong answer: “0.25 moles = 0.25 g, so the mass is 0.25 g.” This scores 0/2. The student has equated moles with grams.

Right answer: “m = n x Mr = 0.25 x 100 = 25 g (1)(1).” This scores 2/2. The formula is used correctly and the answer has the right unit.

How to avoid this mistake

Always write units alongside your numbers. If your answer is an amount, write “mol”. If it is a mass, write “g”. This small habit forces you to distinguish between the two.

Remember: moles is the bridge. You cannot compare masses of different substances directly in a chemical equation. You convert mass to moles (using Mr), use the mole ratio from the balanced equation, then convert back to mass if needed.

The triangle n = m / Mr should be your starting point for any stoichiometry calculation. See the mole and Avogadro constant for full worked examples and relative masses for how to calculate Mr values.