Question

What is the pH of `10^(-3)` M ammonia cyanide solution, if `K_(HCN)=7.2 xx 10^(-11)` and `K_(NH_(3))=1.8 xx 10^(-5)mol L^(-1)` ?
A. 14
B. 9.7
C. `12.0`
D. `7.5`

Answer 1

Correct Answer - B
`[H^(+)]` for the salt of weak acid and weak base is given by the expression
`[H^(+)]=sqrt((K_(w)xx K_(a))/(K_(b)))`
`=sqrt((10^(-14)xx7.2 xx 10^(-11))/(1. 8 xx 10^(-5)))=2 xx 10^(-10)`
`pH =-log 2 xx 10^(-10)=10-log2`
`=10-0.3010=9.699`