HardChemistryHigh School (9–10)GeneralCarbon and Its Compounds › Reactions and Uses of Carbon CompoundsNumerical

Tristearin, a fat of molar mass 890 g/mol, reacts with sodium hydroxide: fat + 3 NaOH → 3 soap +…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Tristearin, a fat of molar mass $890\ \text{g/mol}$ , reacts with sodium hydroxide: fat $+\ 3\,\text{NaOH} \rightarrow 3$ soap $+$ glycerol. A small factory boils $356\ \text{g}$ of the fat. Find the mass of sodium hydroxide needed and the mass of soap made. The soap, sodium stearate, is $306\ \text{g/mol}$ , and NaOH is $40\ \text{g/mol}$ .

Need a reminder? The key ideaShow

Soap is a sodium or potassium salt of a long-chain carboxylic acid. Fat $+\ 3\,\text{NaOH} \rightarrow 3$ soap $+$ glycerol. The tails go into the grease and the heads stay in the water, making micelles. In hard water, calcium and magnesium ions turn soap into scum. Detergents give no scum.

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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← Ethanol reacts with sodium: 2 Na + 2 C₂H₅OH → 2 C₂H₅ONa + H₂.You wash clothes in hard water. With soap, a grey scum forms and there is little lather. →

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