Question

Soaps are sodium salts of fatty acids, RCOONa; e.g. $C_{17}{H}_{35}CO{O}^{-}N{a}^{+}$. It gives an insoluble precipitate / layer with

• A. $C{a}^{2+}$ (aq)
• B. $M{g}^{2+}$ (aq)
• C. HCl (aq)
• D. All of these

Answer 1

The correct option is D All of these

Soaps are sodium salts of fatty acids. It gives an insoluble precipitate/layer with Calcium, Magnesium ions. It also gives an insoluble layer with HCl acid.

As a cleaning agent, soap suffers from two main drawbacks:

• It does not function well in acidic solutions because of the formation of insoluble fatty acid
  

eg, $C{H}_3\left(C{H}_2{)}_{16}CO{O}^{-}N{a}^{+}\right(aq)+HCl(aq)\to C{H}_3(C{H}_2{)}_{16}COOH\left(s\right)+N{a}^{+}\left(aq\right)+C{l}^{-}\left(aq\right)$

• It forms insoluble precipitates with $C{a}^{2+}$ and$M{g}^{2+}$ ions present in hard water, forming a scum
  

eg, $2C{H}_3\left(C{H}_2{)}_{16}CO{O}^{-}N{a}^{+}\right(aq)+C{a}^{2+}(aq)\to [C{H}_3\left(C{H}_2{)}_{16}CO{O}^{-}{]}_{2}C{a}^{2+}\right(s)+2N{a}^{+}(aq)$