General Chemistry 2

Question

Calculate the concentration of HS^- in an aqueous solution of 0.0632 M hydrosulfuric acid, H₂S (aq).

Answer 1

Ka H₂S = 1.0x10^-7 (Wikipedia)

H₂S ==> H⁺ + HS^-

Ka = 1.0x10^-7 = [H⁺][HS^-] / [H₂S]

1.0x10^-7 = (x)(x) / 0.0632 - x

If we assume x is small relative to 0.0632 M, we can ignore it in the denominator and avoid using the quadratic formula.

Thus, we have...

1.0x10^-7 = x² / 0.0632

x² =6.32x10^-9

x = 7.95x10^-5 (this is small relative to 0.0632 so our above assumption was valid)

So, [HS^-] = 7.95x10^-5 M