Chemistry 9701 · AS & A Level · Hydrocarbons

Hydrocarbons — practice question

A gaseous hydrocarbon $A$ with a mass of $0.42\,\text{g}$ is passed slowly over a large excess of heated copper(II) oxide, $\text{CuO}$. $A$ is then completely oxidised. The products are collected, and it is observed that $1.32\,\text{g}$ of $\text{CO}_2$ and $0.54\,\text{g}$ of $\text{H}_2\text{O}$ are produced. Copper is the only additional product formed in the reaction.
(a(i))

Calculate the mass of carbon contained in $1.32\,\text{g}$ of $\text{CO}_2$. Use that result to calculate the amount, in moles, of carbon atoms present in $0.42\,\text{g}$ of $A$.

(a(ii))

Calculate the mass of hydrogen contained in $0.54\,\text{g}$ of $\text{H}_2\text{O}$. Use that result to calculate the amount, in moles, of hydrogen atoms present in $0.42\,\text{g}$ of $A$.

(a(iii))[5]

It is thought that $A$ is an alkene rather than an alkane. Use your answers to (i) and (ii) to deduce whether this is correct. Explain your answer.

(b(i))

A second organic compound, $B$, was analysed and the mass composition was found to be: $\text{C},\;64.86\%$; $\text{H},\;13.50\%$; $\text{O},\;21.64\%$. Use these data to determine the empirical formula of $B$.

(b(ii))

The empirical and molecular formulae of $B$ are identical. $B$ is found to be chiral. Draw displayed formulae of the two optical isomers of this compound, indicating with an asterisk ($*$) the chiral carbon atom.

(b(iii))[7]

There are three further structural isomers of $B$ that are not chiral but do contain the same functional group as $B$. In the boxes below, draw the structural formulae of these isomers.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Carbon mass correctly obtained $= \frac{12 \times 1.32}{44} = 0.36\,\text{g}$

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