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Some compounds with different molecular formulas have the same relative molecular mass to the nearest whole number - AQA - A-Level Chemistry - Question 10 - 2019 - Paper 2
Question 10
Some compounds with different molecular formulas have the same relative molecular mass to the nearest whole number. A dicarboxylic acid has a relative molecular mas... show full transcript
Worked Solution & Example Answer:Some compounds with different molecular formulas have the same relative molecular mass to the nearest whole number - AQA - A-Level Chemistry - Question 10 - 2019 - Paper 2
Step 1
Deduce the molecular formula of the acid.
Answer
The molecular formula of the dicarboxylic acid can be determined by considering its molecular mass. Given the relative molecular mass of 118, one possible dicarboxylic acid with this mass is C₄H₆O₄, which contains two carboxylic acid groups.
Step 2
Calculate the mass, in g, of the dicarboxylic acid used.
Answer
To calculate the mass:
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Determine the amount of NaOH used in moles:
\text{moles of NaOH} = 0.100 ext{ mol/dm}^3 × 0.02160 ext{ dm}^3 = 0.00216 ext{ mol}
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Since the dicarboxylic acid reacts with 2 moles of NaOH, the moles of acid = 0.00216/2 = 0.00108 mol.
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Now, calculate the mass of the acid using:
\text{mass} = \text{moles} × \text{molar mass} = 0.00108 \text{ mol} × 118 \text{ g/mol} = 1.272 \text{ g}
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The answer should be reported to three significant figures: 1.27 g.
Step 3
Deduce the number of peaks in the 'H NMR spectrum of this diol.
Answer
For the given diol with molecular formula C₅H₈O₃, the expected number of peaks in the 'H NMR spectrum is determined by the distinct types of hydrogen atoms present. In the case of this diol, we can expect 4 peaks due to the different environments of the hydrogen atoms.
Step 4
Draw the structure of a different diol also with molecular formula C₅H₈O₃ that has a 'H NMR spectrum that consists of two singlet peaks.
Answer
One possible diol structure with the formula C₅H₈O₃ that would display two singlet peaks in its 'H NMR spectrum is 2,2-dimethyl-1,3-propanediol. Drawing this structure will show two types of hydrogen environments, leading to the observed singlet peaks.
Step 5
State why the dicarboxylic acid can be distinguished from the two diols by high resolution mass spectrometry using electrospray ionisation.
Answer
The dicarboxylic acid can be distinguished from the two diols due to the differences in the fragmentation patterns and charge states observed during high-resolution mass spectrometry. The dicarboxylic acid will produce distinct ions that differ from those formed by the diols, related to the presence of -COOH groups, allowing for clear identification.