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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 deduce the molecular formula of the dicarboxylic acid with a relative molecular mass of 118, we know that a dicarboxylic acid has two carboxyl groups (-COOH). Assuming a hydrocarbon base, the empirical formula can be represented as CₙH₂ₙO₄. Setting up the equation for 118:
The total molar mass becomes:
Solving leads to the molecular formula of C₄H₆O₄.
Step 2
Calculate the mass, in g, of the dicarboxylic acid used.
Answer
To calculate the mass of the dicarboxylic acid used, we first determine the amount of sodium hydroxide (NaOH) that reacted:
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Calculate molarity of NaOH from titration:
- Moles of NaOH = Volume (L) x Concentration (mol/L)
- Moles = 0.02160 L x 0.100 mol/L = 0.00216 moles
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Given that the reaction between NaOH and the dicarboxylic acid is 1:1, the moles of acid used are also 0.00216 moles.
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Calculate mass of the acid using its molar mass:
- Molar mass = 118 g/mol
- Mass = moles x molar mass = 0.00216 moles x 118 g/mol = 0.254 g
- Therefore, mass = 0.254 g.
Step 3
Deduce the number of peaks in the 'H NMR spectrum of this diol.
Answer
In the case of C₄H₈O₄, analyzing the structure shown will reveal how many unique hydrogen environments exist. Each distinct environment corresponds to a peak in the NMR spectrum. Given the symmetrical nature of a diol or similar structure, we find that there are likely 4 unique protons, resulting in 4 peaks in the NMR spectrum.
Step 4
Draw the structure of a different diol also with molecular formula C₄H₈O₄.
Answer
To draw a different diol with the same molecular formula that has two singlet peaks in 'H NMR spectrum, a plausible structure such as:
CH₃
|
H₃C-C-CH
| |
OH OH
This structure may provide the desired singlet peaks due to symmetry.
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 has distinct ionic fragments compared to the two diols. Being able to form [M-H]⁻ ions due to two –COOH groups, the mass spectrometry will show peaks corresponding to these fragments, allowing excellent differentiation between the structures. The low molecular weight of the diol lacks the same fragmentation pattern, providing clear distinctions between them.