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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
To determine the molecular formula of a dicarboxylic acid with a relative molecular mass of 118, we can use the general formula for dicarboxylic acids, which is CₙH₂ₙO₄. By substituting 4 for n (the typical number of carbon atoms), we can calculate:
Adjusting n to find a corresponding formula that gives a molecular mass of 118 suggests the molecular formula is likely C₄H₆O₄.
Step 2
Calculate the mass, in g, of the dicarboxylic acid used.
Answer
To find the mass of the dicarboxylic acid, we first need to determine the number of moles of sodium hydroxide used in the titration:
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Calculate moles of NaOH:
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For dicarboxylic acids, one mole of acid reacts with two moles of NaOH. Thus, the moles of dicarboxylic acid: ext{Moles of acid} = rac{0.00216 ext{ moles NaOH}}{2} = 0.00108 ext{ moles}
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Calculate the mass by using the molar mass of C₄H₆O₄:
Hence, the mass of the dicarboxylic acid used is approximately 127 g when rounded to three significant figures.
Step 3
Deduce the number of peaks in the ¹H NMR spectrum of this diol.
Answer
In ¹H NMR spectroscopy, each unique hydrogen environment produces a peak. The given diol has two -OH groups and multiple alkyl groups that contribute to peaks. As a result, the diol with the formula C₄H₈O₄ will likely have:
- 2 peaks for the -OH hydrogens
- 2 peaks for the hydrogens around the carbon backbone, totaling 4 unique environments.
Thus, the total number of peaks in the ¹H NMR spectrum will be 4.
Step 4
Draw the structure of a different diol also with molecular formula C₄H₈O₄.
Answer
A possible structure for a different diol with the same molecular formula (C₄H₈O₄) could be:
H OH
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H3C-C-C-CH3
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OH H
This structure features two -OH groups positioned on different carbons, leading to distinct chemical environments.
Step 5
State why the dicarboxylic acid can be distinguished from the two diols by high resolution mass spectrometry using electrospray ionisation.
Answer
Dicarboxylic acids can be distinguished from diols through their mass-to-charge ratios (m/z) observed in mass spectrometry. While all compounds (dicarboxylic acid and diols) have the same nominal mass of 118, they will exhibit different fragmentation patterns upon electrospray ionisation due to their unique functional groups (carboxylic acids versus alcohols). The presence of carbonyl (C=O) functional groups in dicarboxylic acids leads to distinct peaks that provide clues regarding their structure, allowing for effective differentiation from diols.