Find the Ka of the weak acid and calculate the pH

[miss88]

So I have the answer for question two and the Ka for question 3, but I do not know how solve it.  Help please?

2.What is the main acid-base equilibrium reaction (the buffer) that maintains the normal physiological pH? What is the weak acid? What is the conjugate base?

ANSWER: weak acid H2CO3 Conjugate base HCO3-
H+(aq) + HCO3-(aq) == H2CO3(aq)

3.Find the Ka of the weak acid and calculate the pH of the buffer when the molarity of the acid is equal to the molarity of the cnojugate base. What is the pH of the buffer when the molarity of the conjugate base is 10 times that of the acid.

Ka for H2CO3 is 4.3x10 -7
pH of blood: (7.35-7.45)

[kingchemist]

If Ka for H2CO3 = [H+][HCO3-]/[H2CO3]

3a.
Then if [HCO3-] = [H2CO3], then Ka = [H+] = 4.3 x 10^-7 mol/L and pH = 6.37

3b.
If Ka for H2CO3 = [H+][HCO3-] x 10/[H2CO3], then Ka = [H+] x 10

so [H+] = 4.3 x 10^-7 mol/L / 10  = 4.3 x 10^-8 mol/L and pH would be 7.37