HardChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsNumerical

1.20 g of ethanoic acid (60.05 g/mol) is dissolved in 50.0 g of benzene…

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$1.20\ \text{g}$ of ethanoic acid ( $60.05\ \text{g/mol}$ ) is dissolved in $50.0\ \text{g}$ of benzene. The freezing point falls by $1.05\ \text{K}$ . For benzene, $K_f = 5.12\ \text{K kg/mol}$ . Find the molar mass from the freezing point depression, the van’t Hoff factor $i$ , and the percentage of the acid that is present as dimer. The dimer forms by $2\,\text{CH}_3\text{COOH} \rightleftharpoons (\text{CH}_3\text{COOH})_2$ .

Need a reminder? The key ideaShow

The carboxyl carbon is carbon 1. Acids come from oxidation of primary alcohols, aldehydes and alkylbenzenes (a benzylic hydrogen is needed), from hydrolysis of nitriles, amides and esters, and from RMgX with CO $_2$ , which adds one carbon. Two hydrogen bonds join acid molecules in a dimer, so the molar mass found in benzene is near double.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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