Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

An aldehyde, an alkane, and a carboxylic acid with broadly similar volatility are combined, and the sample is then run through a gas chromatograph. The resulting gas chromatogram is shown. Their separation is determined by how soluble they are relative to one another in the stationary phase, which is a liquid alcohol.
(a(i))[2]

Complete the table by identifying which substance in the mixture gives peaks X, Y and Z. Justify your choices by referring to the intermolecular forces present in the compounds.

(a(ii))[1]

A student works out the areas under the three peaks on the chromatogram. The area under each peak is proportional to the mass of that compound. Calculate the percentage by mass in the original mixture of the compound that produces peak Z.

(b(i))[1]

A mass spectrum for a halogenoalkane with one chlorine atom or bromine atom shows an extra peak at $M+2$. State the isotopes of chlorine and bromine that are responsible for the $M+2$ peaks.

(b(ii))[2]

The mass spectrum of bromochloromethane, $\text{CH}_2\text{BrCl}$, contains a molecular ion peak, $M$, at an $m/z$ value of $128$. It also displays $M+2$ and $M+4$ peaks. Suggest the identities of the molecular ions responsible for these peaks.

(c(i))[3]

Draw the mechanism for this reaction. Include every relevant curly arrow, dipole and charge.

(c(ii))[1]

1-bromo-2-methylpropane is formed as well. Explain why 2-bromo-2-methylpropane is the major product in this reaction.

(d(i))[2]

Explain the meaning of the term partition coefficient, $K_{\text{partition}}$.

(d(ii))[2]

The partition coefficient for organic compound $\text{H}$ between dichloromethane and water is $4.75$. $2.50\,\text{g}$ of compound $\text{H}$ was dissolved in water and made up to $100\,\text{cm}^3$ in a volumetric flask. $50\,\text{cm}^3$ of this aqueous solution were shaken with $10\,\text{cm}^3$ of dichloromethane. Calculate the mass of compound $\text{H}$ that was extracted into the dichloromethane.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: Peak matching: X = alkane, Y = aldehyde, Z = carboxylic acid

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