Moles calculator: moles, mass or Mr

The mass must be in grams. If a question gives kilograms, multiply by 1,000 first.

• Moles: how many moles are in 9.8 g of sulfuric acid (H₂SO₄)? $M_r = 98$, so $n = 9.8 \div 98 = 0.1$ mol.
• Mass: what is the mass of 0.25 mol of sodium hydroxide (NaOH)? $M_r = 23 + 16 + 1 = 40$, so $m = 0.25 \times 40 = 10$ g.
• Mr: 0.25 mol of a gas has a mass of 11 g. $M_r = 11 \div 0.25 = 44$, which matches carbon dioxide.

A mole is an amount of substance containing $6.02 \times 10^{23}$ particles (the Avogadro constant). One mole of any substance has a mass in grams equal to its $M_r$: one mole of water has a mass of 18 g. That's why the equation works.

Balanced equations tell you the ratio of moles. In $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$, 2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water. Reacting-mass questions usually go: mass to moles, use the ratio, then moles back to mass. See balancing chemical equations.

• Using kilograms instead of grams.
• Dividing the wrong way round ($M_r \div m$).
• Using the wrong $M_r$, often from missing a bracket. Check it with the relative formula mass calculator.

More in moles explained.

1. How many moles are in 4.4 g of carbon dioxide, CO₂ ($M_r = 44$)?
2. What is the mass of 0.2 mol of calcium carbonate, CaCO₃ ($M_r = 100$)?
3. 3 g of a gas is 0.1 mol. What is its $M_r$, and which gas could it be: CH₄ or C₂H₆?

Answers: 0.1 mol; 20 g; $M_r = 30$, so C₂H₆ (ethane).