How can I calculate the mass percent of carbon, nitrogen and oxygen in acetamide, C_2H_5NO?

Dec 12, 2014

The mass percent of a molecule shows the amount each element contributes to the total molecular mass.

The first step in determining the mass percent of a given molecule is to determine each of the molecule component atoms' atomic mass. Since acetamine is composed of carbon, nitrogen, oxygen and hydrogen, we get

$M o l a r$ $m a s {s}_{c a r b o n}$ = 12.0 g/mol
$M o l a r$ $m a s {s}_{n i t r o g e n}$ = 14.0 g/mol
$M o l a r$ $m a s {s}_{o x y g e n}$ = 16.0 g/mol
$M o l a r$ $m a s {s}_{h y \mathrm{dr} o g e n}$ = 1.00g/mol

Each element's mass contribution is determined by multiplying the number of atoms of a certain element with its molar mass

Carbon contributes $2 \cdot 12.0 = 24.0$ g/mol;
Nitrogen contributes $1 \cdot 14.0 = 15.0$ g/mol;
Oxygen contributes $1 \cdot 16.0 = 16.0$ g/mol;
Hydrogen contributes $5 \cdot 1.00 = 5.00$ g/mol

Adding each element's contribution gives us the molecule's total atomic mass

$m o l a r m a s {s}_{{C}_{2} {H}_{5} N O} = 24.0 + 15.0 + 16.0 + 5.00 = 59$ g/mol

The final step to determining the molecul's mass percent is to divide each elemnt's contribution by acetamide's molar mass and to multiply the result by 100%

Mass percent of Carbon = 24.0/59.0 * 100% = 40.7%
Mass percent of Nitrogen = 14.0/59.0 * 100% = 23.7%
Mass percent of Oxygen = 16.0/59.0 * 100% = 27.1%
Mass percent of Hydrogen = 5.00/59.0 = 8.50%

Always check the validity of the answer by adding up all the percentages

40.7 + 23.7 + 27.1 + 8.50 = 100%

So, acetamide is 40.7% Carbon, 23.7% Nitrogen, 27.1% Oxygen, and 8.5% Hydrogen...