Empirical formula from the masses of each element

A-Level (AQA) · unreviewed

Mass of each element in a compound of C, H and O; find its empirical formula.

Worked example

An organic compound contains only carbon, hydrogen and oxygen. A sample contains 2.85 g of carbon, 0.593 g of hydrogen and 0.949 g of oxygen. Its empirical formula has one oxygen atom. How many hydrogen atoms does the empirical formula contain? M(C) = 12.0 g mol⁻¹, M(H) = 1.0 g mol⁻¹, M(O) = 16.0 g mol⁻¹.

1. Moles from mass: solve for n(C)
$n = \dfrac{m}{M}$
= 0.2375 mol
2. Moles from mass: solve for n(H)
$n = \dfrac{m}{M}$
= 0.5930 mol
3. Moles from mass: solve for n(O), the smallest
$n = \dfrac{m}{M}$
= 0.05931 mol
4. Divide n(H) ÷ n(O), the smallest → n(H) ÷ n(O)
= 9.998

Answer: H atoms in the empirical formula = 10.0

Method

Convert each mass to moles (n = m/M), divide every amount by the smallest (here oxygen), and round to whole numbers. The Table in + Operation does this in one go: a row of masses, a row of molar masses, n = m/M, ÷ smallest, then Round.

Common mistakes

That is a ratio of masses. Convert each mass to moles first; the formula is a ratio of atoms.
That is the number of carbon atoms. The question asks for hydrogen.

Formulas used

Moles from mass
$n = \dfrac{m}{M}$

Practise this with fresh numbers

Empirical formula from the masses of each element

Worked example

Method

Common mistakes

Formulas used

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