Question

1)What is the pH of a saturated solution of lead(II) hydroxide? The Ksp= 4.0e-15

Pb(OH)2

4.0e-15 = (x)(2x)^2 = 4x^3

I divided by 4 and took the cube root and got 1.12e-5 but I am not sure I took the cube root correctly.

-log(1.12e-5)= 4.95
14-4.95 = 9.05

The answer choices given are 9.0 and 9.1.

Did I do this correctly?

2) Which pair of mixtures is the most effective buffer system for a pH value of 7.45?

a) Carbonic acid and sodium bicarbonate
b) Sodium bicarbonate and sodium carbonate
c) Sodium dihydrogen phosphate and disodium hydrogen phosphate
d) Hydrogen cyanide and sodium cyanide
e) Ammonia and ammonium chloride

I chose answer c, sodium dihydrogen phosphate abd disodium hydrogen phosphate.

Would you agree?

Thank you for checking my answers.

Answer 1

Thanks for showing your work.
Your 4x^3 = 4.0E-15 is ok. Somewhere in the next two steps you made a math error.
Then x^3 = 4.0E-15/4 = 1E-15
cube root (1E-15) = 1E-5 = x = (OH^-)
Then pOH = 5 and pH = 14-5 = 9

I didn't look up the pK2 value for H3PO4 but I think you are right with c.