The ionization constant of chloroacetic acid is `1.35 × 10^(-3)`. What will be the pH of 0.1M acid and its 0.1M sodium salt solution?

Explanation

pH of 0.1 M acid
Suppose the degree of ionisations of chloroacetic acid is ` alpha `.
` {:(, CH_2ClCOOH , hArr , CH_2ClCOO^(-) , + ,H^(+) ),(" Initial conc. " , 0.1M,, -,,- ),(" Conc.at.eq",0.1(1-alpha),,0.1xx alpha,, 0.1 xx alpha):}`
` therefore " "K_a= ( [CH_2ClCOO^(-) ][H^(+) ])/([CH_2ClCOOH])`
`or " " 1.35 xx 10 ^(-3) =(0.1alpha xx 0.1 alpha)/(0.1(1-alpha))~~((0.1alpha)^(2))/(0.1)`
` or " " alpha = ((1.35xx 10^(_3) xx 0.1)/((0.1)^(2))) ^(1//2) = 0.116`
` therefore " " [H^(+)]=0.1xxalpha = 0.1 xx 0.116= 0.0116`
and ` " "pH = -log_(10) [H^(+) ] =- log_(10) (0.0116) =1.94`
pH of 0.1 M Sodium salt:
The sodium salt of chloroacetic acid is the salt of a weak acid and a strong base For such salt .
`pH =(1)/(2) pK_w +(1)/(2) pK_a +(1)/(2) log_(10) C `
`= (1)/(2) (- log_(10) 1.0 xx 10^(-14) )+(1)/(2) (-log_(10) 1.35 xx 10^(-3)) +(1)/(2) log_(10) 0.1 `
` = 7.0 +1.435 +(-0.5) = 7.94`