Question

How can you calculate the relative molecular mass and percentage composition of a compound

Answer 1

Relative molecular mass:

• The sum of all the relative atomic masses of the atoms in a molecule; the ratio of the average mass per molecule of a specified isotopic composition of a substance to one-twelfth the mass of an atom of carbon-12.
• Symbol:${\mathrm{M}}_{\mathrm{r}}$, Abbreviation: r.m.m. Also known as molecular weight.

Percentage composition of a compound:

• The percentage composition of a given compound is defined as the ratio of the amount of each element to the total amount of individual elements present in the compound multiplied by 100.
• So, 2 hydrogen moles weigh 2.016 grams. Hence, one mole of water has 2.016 grams of hydrogen mole. Therefore, the percentage composition of hydrogen would be 2.016/18.0152 = 11.19%.

For example, Carbon dioxide has the formula ${\mathrm{CO}}_2$ [$\mathrm{C}=12,\mathrm{O}=16$]

Molecular weight=$12+2\times 16=44$

Percentage composition of each element:-

$\mathrm{C}$=$\frac{12}{44}\times 100=27.27\%$

$\mathrm{O}$=$\frac{32}{44}\times 100=72.72\%$

$\therefore$The relative molecular weight of carbon dioxide is 44 and the percentage of carbon and oxygen in carbon dioxide are 27.27% & 72.72% respectively