The relative formula mass of a compound is calculated by adding together the relative atomic mass values for all the atoms in its formula. Moles are units used to measure substance amount.

This equation shows how relative formula mass, number of moles and mass are related:

number of moles = mass ÷ relative formula mass

This can be rearranged to find the mass if the number of moles and **molar mass** (its relative formula mass in grams) are known. It can also be rearranged to find the molar mass if the mass and number of moles are known.

The triangle diagram may help you with this.

- Question
Calculate the number of moles of carbon dioxide molecules in 22 g of CO

_{2}.A

_{r}(relative atomic mass) of C = 12, A_{r}of O = 16M

_{r}(relative formula mass) of carbon dioxide = 12 + 16 + 16 = 44number of moles = 22 ÷ 44 =

**0.5 mol**

- Question
Calculate the mass of 2 mol of carbon dioxide (CO

_{2}).mass = number of moles × relative formula mass = 2 × 44 =

**88 g**

- Question
10 mol of carbon dioxide has a mass of 440 g. What is the relative formula mass of carbon dioxide?

relative formula mass = mass ÷ number of moles = 440 ÷ 10 =

**44**

You can calculate the mass of a product or reactant using the idea of moles, a balanced equation and relevant A_{r} values.

Sulfuric acid and sodium hydroxide react together to make sodium sulfate and water:

H_{2}SO_{4} + 2NaOH → Na_{2}SO_{4} + 2H_{2}O

- Question
Calculate the

**mass of sodium sulfate**made when 20 g of sodium hydroxide reacts with excess sulfuric acid. (A_{r}of H = 1, A_{r}of O = 16, A_{r}of Na = 23, A_{r}of S = 32)M

_{r}of NaOH = 23 + 16 + 1 = 40M

_{r}of Na_{2}SO_{4}= 23 + 23 + 32 + 16 + 16 + 16 + 16 = 142Number of moles of NaOH = mass ÷ relative formula mass = 20 ÷ 40 = 0.5 mol

From the equation, 2 mol of NaOH reacts with 1 mol of Na

_{2}SO_{4}, so 0.5 mol of NaOH will react with 0.25 mol of Na_{2}SO_{4}.mass of Na

_{2}SO_{4}= moles × relative formula mass = 0.25 × 142 =**35.5 g**

The example above could also be tackled like this:

\[mass \enspace of \enspace Na_{2}SO_{4} = \frac{20}{2 \times 40} \times 142 = 35.5g\]