Question

A solution contains 0.0010 M each of Ca2+, Sr2+, and Ba2+, What is the concentration of sulfate ion needed to precipitate all of strontium ion and barium ion but NOT calcium ion? Ksp: CaSO4 6.1 x 10-5 SrSO4 3.2 x 10-7 BaSO4 1.5 x 10-9

A) larger than 3.2 x 10-4 M but smaller than 6.1 x 10-2 M

B) 0.001 M

C) smaller than 1.5 x 10-6 M

D) larger than 1.5 x 10-6 M but smaller than 3.2 x 10-4 M

Please show all work and provide a full explanation as to how correct answer is obtained, thank you.

Answer 1

for CaSO4 precipitation:
Ksp = [Ca2+] [so42-]
6.1*10^-5 = 0.001*[SO42-]
[SO42-] = 0.061 M
to not precipitate this ion,
[SO42-] < 0.061 M

for SrSO4:
Ksp = [Sr2+] [SO42-]
3.2*10^-7 = 0.001*[SO42-]
[SO42-] =3.2*10^-4 M
to precipitate SrSO4,
[SO42-] > 3.2*10^-4 M

for BaSO4:
Ksp = [Ba2+] [SO42-]
1.5*10^-9 = 0.001*[SO42-]
[SO42-] =1.5*10^-6 M
to precipitate BaSO4,
[SO42-] > 1.5*10^-6 M

3 conditions are satisfied for:
A) larger than 3.2 x 10-4 M but smaller than 6.1 x 10-2 M
Answer: A