Problem 1
K_a1 = 6.0x10^-4    K_a2 = 6.4x10^-7
Calculate pH after mixing 50.0 mL of 0.10 M H₂A and 30.0 mL of 0.10 M NaOH.
mol H₂A = M^.L = 0.0050
mol NaOH = M^.L = 0.0030
After mixing, 0.0020 mol H₂A remain and 0.0030 mol HA^-1 form.
{Assume 50 HHA allowed to react with 30 NaOH.  The 50 H (Ka1) react first. At equilibrium, 20 H and 30 HA^-1 (CB) remain.}
In:  0.00500mol
    H₂A  H⁺¹  +  HA^-1
Eq:  0.00200mol/0.080L    ?             0.00300mol/0.080L

K_a1 = [H⁺¹][HA^-1]/[H₂A] = [H⁺¹][0.0030]/[0.0020] = 6.0x10^-4
[H⁺¹] = 4.0x10^-4     pH = 3.40
 
 

Problem 2
K_a1 = 6.0x10^-4    K_a2 = 6.4x10^-7
Calculate pH after mixing 50.0 mL of 0.10 M H₂A and 70.0 mL of 0.10 M NaOH.
mol H₂A = M^.L = 0.0050
mol NaOH = M^.L = 0.0070
After mixing, H⁺¹ from Ka1 completely neutralized.
0.0030 mol HA^-1 remain and 0.0020 mol A^-2 form.
{Assume 50 HHA allowed to react with 70 NaOH.  The 50 H (Ka1) react first. At equilibrium, 30 HA^-1 and 20 A^-2 (CB) remain.}
In:  0.00500mol
    HA^-1 H⁺¹  +  A^-2
Eq:  0.0030mol/0.12L         ?         0.0020mol/0.12L

K_a2 = [H⁺¹][A^-2]/[HA^-1] = [H⁺¹][0.0020]/[0.0030] = 6.4x10^-7
[H⁺¹] = 9.6x10^-7     pH = 6.02