I need help with chemistry. I want to check my answer?

Question

1) Calculate the pH of .00042 M HAc
Are there 2 answers?
Is the answer -3.66 and -5.45?
2) Mixture of 0.080 M NaCH3CO2 with 0.030 M HAc pH=?
(I need help with this one)

Answer 1

1. Here, H+ ion concentration is 4.2*10^-4;
and pH = -log (H+);
or pH = - log (4.2*10^-4);
or pH = -log (4.2) + 4 log 10;
or pH = -0.6232 + 4.00;
or pH = 3.3768;
2. pH of buffer solution :

pH =pKa - log (Ac/HAc);

or pH = 4.76-log (0.080/0.030);
or pH = 4.76- 0 .4260;
or pH = 4.334
pH of the mixture =4.334

Answer 2

The acid dissociation constant for acetic acid is 1.74*10^-5. The dissociation reaction is

HAc <------> [H+] + [Ac-] let x = [H+], then [HAc] after dissociation is [C] - x ([C] is the initial concentration of 0.00042 M). From the equation,

[C] - x <-------> x + x

The equilibrium relation is Ka = x^2 / ([C] - x)

Ka*([C] - x) = x^2, x^2 + Ka*x - Ka*[C] = 0

Solve with the quadratic formula, x = 7.72*10^-5.

x = [H+], pH = -log[H+], so pH = -log(x) = 4.11 (only positive values allowed for pH).

The second is a buffer solution defined by the Henderson-Hasselbalch equation:

(Note: CH3CO2- is the acetate Ac anion)

pH = pKa - log( [Ac-]/[HAc] )

[Ac-] = 0.080 M (all the anion comes from the salt--see the reference)
[HAc] = 0.030 M

pKa = -log(Ka) = 4.76

pH = 4.33

Answer 3

I will give you the first part

1. pH = -log(Concentration of acid)