Relative formula mass (Mr) calculator

1. Count the atoms of each element in the formula.
2. Multiply each count by that element's $A_r$ from the periodic table.
3. Add them up.

Example: H₂SO₄

• H: $2 \times 1 = 2$
• S: $1 \times 32 = 32$
• O: $4 \times 16 = 64$

$M_r = 2 + 32 + 64 = 98$.

A number after a bracket multiplies everything inside it. In Ca(OH)₂ there are 2 oxygen atoms and 2 hydrogen atoms:

$M_r = 40 + 2 \times (16 + 1) = 40 + 34 = 74$.

These use the relative atomic masses on the AQA GCSE periodic table, where chlorine is 35.5 and copper is 63.5. Some boards round copper to 64, so always use the periodic table your exam gives you.

To find the percentage of an element in a compound:

For carbon in CO₂: $\dfrac{12}{44} \times 100 \approx 27.3\%$.

$M_r$ is what links mass to moles: $\text{moles} = \dfrac{\text{mass (g)}}{M_r}$. Try the moles calculator, or read relative formula mass explained and moles explained.

1. What is the $M_r$ of ammonia, NH₃?
2. What is the $M_r$ of ammonium sulfate, (NH₄)₂SO₄?
3. What percentage of magnesium oxide, MgO, is magnesium by mass?

Answers: 17; $2 \times (14 + 4) + 32 + 64 = 132$; $24 \div 40 \times 100 = 60\%$.