Question

Find the pH of each of the following solutions of mixtures of acids.

A)0.115 M in HBr and 0.120 M in HCHO2
Express your answer to three decimal places.

B)0.165 M in HNO2 and 9.0×10−2 M in HNO3
Express your answer to two decimal places.

Answer 1

HCOOH is weak acid having pKa of 3.75, HBt is strong acid and ionizes completely.

HBr-------->H+ + Br- , [H+] = 0.115

HCOOH------->HCOO- + H+

let x= drop in concentration of HCOOH to reach equilibrium

at Equilibirum [H+] = [HCOO-]= x and [HCOOH] =0.120-x

Ka= [HCOO-][H+]/[HCOOH] =10^(-3.75)= 0.000178

x²/(0.12-x)= 0.000178, when solved using excel, x=0.00452, [H+]total = 0.00452+0.1115= 0.11952

pH= -log[H+]= 0.92

2. Ka of HNO2= 7.1*10-4, HNO2 is weak acid

HNO2------->H+ + NO2-

Ka= 7.1*10-4 = [H+][NO2-]/[HNO2]

let x= drop in concentration of HNO2 to reach equilibirum

at equilibrium [HNO2] = 0.165-x, 7.1*10-4= x²/(0.165-x) , when solved using excel, x= 0.0105

HNO3 is strong acid and hence [H+] from HNO3= 9/100 =0.09M

total [H+] =0.09+0.0105= 0.1005, pH= -log(0.1005)= 0.998