Moles and masses - Higher

The mass of a substance can be calculated from the number of moles, and the number of moles of a substance can be calculated from its mass. The link between the two quantities is the Mr, relative formula mass (or Ar, relative atomic mass for atoms and metals).

Calculating masses

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The mass of a given number of moles of substance is calculated using: mass~in~g = \textit{M}_{\textup{r}}\times~number~of~moles~or~mass =\textit{M}_{\textup{r}}~\times~mol

One way to remember this is 'mass equals mister mole'.

Example

Calculate the mass of 0.25 mol of carbon dioxide molecules . (Mr of CO2 = 44)

mass = 44 × 0.25

= 11 g

The calculation is the same if a substance is a metal or exists as separate atoms, but its Ar is used instead of an Mr.

Question

Calculate the mass of 0.10 mol of iron. (Ar of Fe = 56)

mass = 56 × 0.10

= 5.6 g

Calculating moles

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The amount of a given mass substance is calculated using: amount~in~mol=\frac{mass~in~g}{\textit{M}_\textup{r}}

Again, use Ar instead of Mr for metals or a separate atom.

Example

Calculate the amount of carbon atoms in 6.0 g of carbon. (Ar of C = 12)

amount = \frac{\textup{mass}}{{A}_\textup{r}}

amount =\frac{\textup6.0}{\textup12}

= 0.5 mol