A1. Analytical techniques are very useful in determining molecular structures. A compound, X, has the empirical formula C2H4O.
(a) Identify the analytical technique that would most readily provide the additional data required to calculate the molecular formula of X.
(b) The molecular formula of X is C4H8O2. The information in the IR spectrum below can be used to help determine the structure of X.
(i) State what information about a molecule can be obtained from its IR spectrum.
(ii) Deduce the information obtained from absorptions A and B.
(iii) Comment on the absence of any major absorption in the region 1700–1750 cm–1.
(c) The 1H NMR spectrum of X shows three peaks with relative areas of 2:1:1.
(i) Deduce what information can be obtained from these data.
(ii) Deduce the structure of X.
A2. Atomic absorption spectroscopy is frequently used to determine the concentration of metal ions in aqueous solution.
(a) Atomic absorption spectroscopy was used to determine the concentration of magnesium in cast iron. Solutions of known concentration were used for calibration.
(b) When a 0.200 g sample of cast iron was dissolved in acid to produce 100 cm3 of solution, the absorbance of the resulting solution was 0.131. Calculate the percentage by mass of magnesium in the sample.
(c) The same sample is also analysed for calcium. Identify the change to the instrument that is required.
A3. Chromatography is a technique frequently used to separate mixtures of substances.
(a) Describe how you would show that a sugar solution was a mixture of two substances using the technique of thin-layer chromatography (TLC).
(b) Given solutions of glucose and fructose, explain how you would show that these were the two sugars present in the mixture.