Question

A 2.800g sample of an unknown compound that contains only carbon and hydrogen is burned in excess oxygen. The water vapour formed in the complete combustion reaction has a mass of 3.600g. In a separate experiment the molar mass of the unknown compound was determined to be 84.0 g/mol. Determine the empirical and molecular formula of the unknown compound.

Answer 1

Question # 59835, Chemistry / General Chemistry | for completion

A 2.800g sample of an unknown compound that contains only carbon and hydrogen is burned in excess oxygen. The water vapour formed in the complete combustion reaction has a mass of 3.600g. In a separate experiment the molar mass of the unknown compound was determined to be 84.0 g/mol. Determine the empirical and molecular formula of the unknown compound.

Answer

2.8 g 3.6 g

$$C _ {x} H _ {y} + O _ {2} \rightarrow CO _ {2} + H _ {2} O$$

Mr 18 g/mol

$$\operatorname {Mr} \left(\mathrm {C} _ {\mathrm {x}} \mathrm {H} _ {\mathrm {y}}\right) = \frac {2 . 8 \cdot 1 8}{3 . 6} = 1 4 \mathrm{~g/mol}$$

The simplest formula has molecular weight 14 g/mol and it is CH₂

$$n = \frac {M r _ {\text {(c o m p o u n d)}}}{M r \left(C H _ {2}\right)} = \frac {8 4}{1 4} = 6$$ $$\mathrm{CH_2 \cdot 6 = C_6H_{12}}$$

**Answer**: The empirical formula of the unknown compound is $\mathbf{CH}_2$, whereas molecular formula is $\mathbf{C}_6\mathbf{H}_{12}$.

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