CAIE A-Level Chemistry 2.3.5 Empirical and Molecular Formula Calculations

Practise converting composition data to mole ratios and using relative mass to obtain a molecular or hydrate formula.

Exam points

convert each element’s mass or percentage to moles before finding the smallest whole-number ratio
multiply all ratios consistently when division gives simple fractions rather than rounding prematurely
use molecular Mr divided by empirical-formula mass to scale the empirical formula to the molecular formula

[Maximum number: 3]

Compound V is a liquid.
V contains 77.2 % carbon, 11.4 % hydrogen and 11.4 % oxygen by mass.
V has a relative molecular mass of 280 .

Calculate the molecular formula of V. Show your working.
molecular formula of V=

Calculation:

element	percentage / $A_{\mathrm r}$	divide by 0.7125	ratio
C	77.2/12=6.433	6.433/0.7125	9.03
H	11.4/1=11.4	11.4/0.7125	16
O	11.4/16=0.7125	0.7125/0.7125	1

Empirical formula: $\mathrm{C}_{9}\mathrm{H}_{16}\mathrm{O}$ .
$M_{\mathrm r}(\mathrm{C}_{9}\mathrm{H}_{16}\mathrm{O})=140$ .
Since 280/140=2, the molecular formula of V is $\mathrm{C}_{18}\mathrm{H}_{32}\mathrm{O}_{2}$ .

\mathrm{C}_{18}\mathrm{H}_{32}\mathrm{O}_{2}

M1: $\% / A_{\mathrm r}$ for C, H and O.
M2: divide each value by the smallest value to give the simplest whole number ratio / empirical formula.
M3: compare $M_{\mathrm r}$ from the M2 ratio with 280 to deduce the molecular formula.

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2.3.5—Empirical and molecular formulas question 1