Question

AUSWC. 5) What is the H30* concentration to the correct number of significant figures for solutions with the following pH values? (a) 0.00 (b) 7.00 (C) 2.63 (d) -0.42 (e) 11.21 (a) Calculate the pH of a buffer solution that is 0.45 M in NH4Cl and 0.15 M in NH3. (b) How would you prepare an NH4Cl-NH3 buffer that has a pH of 9.00? (The Kb for NH3 is 1.8 x 10)

Answer 1

5) we know that, pH= -log [H+] so,[H+] = 10^-pH

a) pH = 0.0 so, [H+] = 10^-0.0 = 1.0 M

b) pH = 7.00 so [H+] = 10^ -7.00 = 1.0*10^-7 M

c) pH = 2.63 so [H+] = 10^-2.63 = 2.34*10^-3 M

d) pH = -0.42 so [H+] = 10^0.42 = 2.63 M

e) pH = 11.21 so pOH = 14-11.21 = 2.79 so [OH-] = 10^-2.79 = 1.62*10^-3 M

now [H+] * [OH-] = 10^-14 so [H+] = 6.17*10^-12 M

6)a) pOH = pKb + log [salt]/[base]

pKb = -log Kb = -log 1.8*10^-5 =4.74

so pOH = 4.74 + log 0.45/0.15 = 4.74+log 3 = 4.74+0.48 = 5.22

so pH = 14-5.22 = 8.78

b) pH of the buffer is 9.00

so pOH = 5.00

according to the equation log [salt]/[base] = 5-4.74 = 0.26

[salt] / [base] = 10^0.26 = 1.82

so for preparation of a buffer of pH 9 the ratio of concentration of NH4Cl and NH3 must be equal to 1.82