1.

When NaOH solution is gradually added to the solution of weak acid (HA), the pH of the solution is found to be 5.0 at the addition of 10and 6.0 at further addition of 10 ml of same NaOH. (Total volume of NaOH =20ml). Calculate pK_(a) for HA. [log2 = 0.3]

Answer»


Solution : Let INITIAL com HA and NAOH be `c_(1)` and `c_(2)` and initial volume of `HA=v_(1)ML`
`5=pK_(a) + "LOG"(10C_(2))/((c_(1)v_(1) – 10c_(2)))` ……(i)
`6=pK_(a) + "log"(20C_(2))/((c_(1)v_(1) – 20c_(2)))` ……(ii)
`c_(1)v_(1)=22.5c_(2)`
`5=pK_(a) + "log"(10c_(2))/(22.5c_(2) – 10c_(2)) = pK_(a) + log0.8 IMPLIES pK_(a)=5+0.096=5.1`


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