Question

In a qualitative analysis procedure, a chemist adds HCI to an unknown group of ions and then saturates the solution with H₂S so that the solution is 0.4 M in HCI and 0.10 M in H₂S 

(a) What is the [HS-] of the solution? (Note: The answer should be rounded to one significant figure. Be careful, 3.5 rounds up to 4, but 4.5 rounds down to 4) 

(b) If 0.01 M of each of the following ions is in the solution, which will form a precipitate? (Select all that apply.)

Answer 1

a)

Given data,

[H⁺] = 0.4 M

[H₂S] = 0.1 M

We know that,

K_a of H₂S = 1 x 10^-7

Let us consider a reaction,

H₂S ------> H⁺ + HS^-

HS^- = K_a[H₂S] / [H⁺]

= 1 x 10^-7 x 0.1 / 0.4

HS^- = 2.5 x 10^-8

b)

Pb²⁺ and Cu²⁺ ions will precipitate