Chemistry · General chemistry I · Worked example

Find percent composition from a formula

Urea, CO(NH₂)₂, is a common fertilizer. What percent of its mass is nitrogen?

Count the atoms

CO(NH₂)₂ has one carbon, one oxygen, two nitrogen and four hydrogen atoms: the subscript 2 after the parentheses doubles everything inside them.

Add the atomic masses with their counts: 12.01 + 16.00 + 2(14.01) + 4(1.008) = 60.06 g/mol.

M (CO {({NH}_{2})}_{2}) = 12.01 + 16.00 + 2 (14.01) + 4 (1.008) = 60.06 g/mol

\begin{aligned} M (CO {({NH}_{2})}_{2}) & = 12.01 + 16.00 \\ + 2 (14.01) + 4 (1.008) \\ = 60.06 g/mol \end{aligned}

One mole of urea contains 2(14.01) = 28.02 g of nitrogen. Dividing by 60.06 g and multiplying by 100% gives 46.65%.

\frac{28.02 g N}{60.06 g} \times 100 % = 46.65 %

Carbon is 20.00%, oxygen 26.64% and hydrogen 6.71% of the mass. With nitrogen, they add to 100.00%, which checks the arithmetic.

Urea is 46.65% nitrogen by mass.

What is the mass percent of oxygen in water?

Show the answer and explanation

M(H₂O) = 18.016 g/mol, of which 16.00 g is oxygen. 16.00/18.016 × 100% = 88.81%.

Enter another fertilizer, such as NH4NO3, in Composition & formulas and compare its nitrogen percentage.

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