how to find mass number

Formula mass and mole calculations

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.

Mole calculations

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.

Triangle divided into three with mass in top section, moles and below left and molar mass below right, with an 'x' between them

Finding the number of moles

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 = 16

M _r (relative formula mass) of carbon dioxide = 12 + 16 + 16 = 44

number of moles = 22 ÷ 44 = 0.5 mol

Finding the mass

Question: Calculate the mass of 2 mol of carbon dioxide (CO ₂ ).
: mass = number of moles × relative formula mass = 2 × 44 = 88 g

Finding the relative formula mass

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

Reacting masses

You can calculate the mass of a product or reactant using the idea of moles, a balanced equation and relevant A _r values.

Example

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 = 40

M _r of Na ₂ SO ₄ = 23 + 23 + 32 + 16 + 16 + 16 + 16 = 142

Number 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:

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