1:28 understand how to carry out calculations involving amount of substance, relative atomic mass (Aᵣ) and relative formula mass (Mᵣ)

The following formula allows for the interconversion between a mass in grams and a number of moles for a given substance:

Amount(mol) = \frac{mass(g)}{M_r}

Example 1:

Calculate the amount, in moles, of 8.8 g of carbon dioxide (CO₂).

Step 1: Calculate the relative formula mass (M_r) of carbon dioxide (CO₂).

Carbon\,dioxide = CO_2 Atoms\,present = (1 \times C) + (2 \times O) M_r = (1 \times 12) + (2 \times 16) = 44

Step 2: Use the formula to calculate the amount in moles.

Amount = \frac{mass(g)}{M_r} Amount = \frac{8.8}{44} Amount = 0.2 mol

Example 2:

Calculate the mass of 2 mol of copper(II) sulfate (CuSO₄).

Step 1: Calculate the relative formula mass (M_r) of copper(II) sulfate (CuSO₄).

copper(II)\,sulfate = CuSO_4 Atoms\,present = (1 \times Cu) + (1 \times S) + (4 \times O) M_r = (1 \times 63.5) + (1 \times 32) + (4 \times 16) = 159.5

Step 2: Rearrange the formula to calculate the mass.

mass = amount \times M_r mass = 2 \times 159.5 mass = 319g