Question

What is the pH of a .20 M MgCO₃ solution

What is the pH of a .20 M NaF solution

Answer 1

A)

concentration of MgCO3= 0.20M

MgCO3 -------------- Mg+2 + CO3-2

CO3-2    + H2O ------------------- HCO3-   + OH-

0.20 M                                        0M              0M

-x                                               +x                 +x

0.20-x                                          +x                  +x

kb= [HCO3-][OH-]/[CO3-2]

Ka of H2CO3= 4.3x10&-7

KaxKb= Kw                      where Kw= 1.0x10^-14

Kb= Kw/Ka = 1.0x10^-14/4.3x10^-7 = 2.33x10^-8

Kb= 2.33x10^-8

2.33x10^-8 = x*x/(0.20-x)

for solving the eqaution

x= 6.825x10^-5

[OH-] = 6.825x10^-5 M

-log[OH-]= -log( 6.825x10^-5)

POH=4.16

PH+POH=14

PH= 14 -POH

PH= 14- 4.16

PH= 9.84.

B)

concentration of NaF = 0.20M

NaF ------------------ Na+ + F-

F- + H2O ------------------- HF + OH-

0.20                                  0          0

-x                                      +x       +x

0.20-x                              +x           +x

ka of HF = 7.2x10^-4

Kb= 1.0x10^-14/7.2x10^-4 = 1.39x10^11

Kb = [HF][OH]/[F-]

1.39x10^-11 = x*x/(0.20-x)

for solving the equation

x= 1.67x10^-6

[OH-] = 1.67x10^-6

-log[OH-] = -log( 1.67x10^-6)

POH= 5.78

PH= 14-5.78

Ph = 8.22