High School Chemistry  ·  Practice by Skill

Mole Conversions Practice Problems

The mole is the counting unit chemists use to link tiny particles to masses you can weigh. Nearly every calculation later in chemistry starts with a mole conversion, so it is worth making these quick and reliable. Practice below covers molar mass, grams to moles to particles, percent composition, and empirical formulas.

What you’ll practice

  • Molar mass: add up atomic masses from the periodic table to get grams per mole.
  • Grams, moles, and particles: convert in either direction using molar mass and Avogadro's number.
  • Percent composition: find the percent by mass of each element in a compound.
  • Empirical and molecular formulas: turn mass or ratio data into the simplest whole-number formula.

Learn the method first →

A quick example: grams to molecules

How many molecules are in 36.0 g of water? The molar mass of H2O is 18.02 g/mol.

Grams to moles: 36.0 g ÷ 18.02 g/mol = 1.998 mol.

Moles to molecules: 1.998 mol × 6.022 × 1023 molecules/mol.

That is 1.20 × 1024 molecules of H2O.

3 free mole conversions problems

Give each one a real try before you open the solution. Your answer is checked as soon as you submit it, and the worked solution shows every step so you can see exactly where a setup went wrong.

#1Gram–Mol
How many moles are in 45.0 g of water (H2O)? (Molar mass of H2O = 18.015 g/mol)
#2Mole Concept
Two different samples each contain exactly 1.00 mol of their respective compounds. Which statement is correct?
#3Empirical Formula
Combustion analysis of a hydrocarbon shows that it contains only carbon and hydrogen in a 1:2 mole ratio (C:H). What is the empirical formula of this compound?

Questions you might have about mole conversions

What is a mole in chemistry?
A mole is 6.022 × 1023 particles of anything, whether atoms, molecules, or formula units. It works like a dozen, but for counting particles too small to count one at a time.
How do I convert grams to moles?
Divide the mass in grams by the molar mass in grams per mole. To go back from moles to grams, multiply by the molar mass instead.
How do I find molar mass?
Multiply the atomic mass of each element by the number of atoms of it in the formula, then add. For CO2 that is 12.01 + 2(16.00) = 44.01 g/mol.
How do I find an empirical formula?
Convert each element's mass to moles, divide all the mole values by the smallest one, and round to whole numbers. Those whole numbers become the subscripts.
High School Chemistry · Mole Conversions Practice Problems