(i) How would you calculate the solubility of sparingly soluble salt using Kohlrausch's law ?
(ii) Formic acid act as reducing agent. Prove this statement.

Explanation

(i) Substances like `AgCl, PbSO_(4)` are sparingly soluble in water. The solubility product can be determined using conductivity experimentss.
2. Let us consier AgCl as an example
`AgCl_((s)) harr Ag^(+) + Cl^(-)`
`K_(SP) = [Ag^(+)] [Cl^(-)]`
3. Let the concentration of `[Ag^(+)]` be .C. mol `L^(-1)`
If `[Ag^(+)]` = C, then `[Cl^(-)]` is also equal to C mol `L^(-1)`
`therefore K_(sp) = C.C`
`K_(sp) = C^(2)`
4. The relationship between molar conductance and equivalent conductance is
`A_(@) = (K xx 10^(-3))/(C(mol L^(-1)))(or) C = (K xx 10^(-3))/(A^(@))`
Substitute the concentration value in the relation `K_(sp) = C^(2)`
`K_(sp) = [(K xx 10^(-3))/(A^(@))]^(2)`
(ii) 1. Formic acid contains both an aldehyde as well as an acid group. Hence, like other aldehydes, formic acid can easily be oxidised and therefore acts as a strong reducing agent.

2. Formic acid reduces Tollen.s reagent (ammonical silver nitrate solution) to metallic silver.
`HCOO^(-) + underset(("Tollen.s reagent"))(2 Ag^(+) + 3OH^(-)) rarr underset(("Silver mirror"))(2Ag + CO_(3)^(2-)) + 2H_(2)O`
3. Formic acid reduces Fehling.s solution. It reduces blue coloured cupric ions to red coloured cuprous ions.
`HCOO^(-) + underset(("Blue"))underset(("Fehlings solution"))(2Cu^(2+) + 5OH^(-)) rarr underset("Red precipitate")(Cu_(2)O darr) + CO_(3)^(2-) + 3H_(2)O`