Solid Na2SO4 is added to a solution which is 0.012 M in Pb(NO3)2 and 0.048 M in AgNO3. Assume the volume remains constant. Ksp = 2.0 10-8 for PbSO4 and Ksp = 1.2 10-5 for Ag2SO4.
(a) Which compound precipitates first?
(b) What is the concentration of the first ion precipitated when the second ion starts to precipitate?
Hi Gregory.
Well, what a coincidence. Hello Stephon! My uncle's name is Stephon, but he lives across the country from me. Anyways, can someone please help me with this problem?
At least with part b. I figured out that part a is Pb(NO3)2
Lol same boat buddy.
Here is the same problem (but different numbers) I worked for John just below.
http://www.jiskha.com/display.cgi?id=1363319855
Thank you so much once again Dr. Bob!
To determine which compound precipitates first, we need to compare the ion concentrations with their respective solubility product constants (Ksp). The compound with a smaller ion concentration compared to its Ksp will precipitate first.
(a) Let's calculate the ion concentrations for PbSO4 and Ag2SO4:
For PbSO4:
[Pb2+] = 0.012 M (from Pb(NO3)2)
[SO4^2-] = x (unknown concentration)
For Ag2SO4:
[Ag+] = 0.048 M (from AgNO3)
[SO4^2-] = x (unknown concentration)
Using the solubility product constants, we can write the following equations:
PbSO4: [Pb2+][SO4^2-] = Ksp(PbSO4)
Ag2SO4: [Ag+][SO4^2-] = Ksp(Ag2SO4)
Plugging in the given Ksp values:
Ksp(PbSO4) = 2.0 x 10^-8
Ksp(Ag2SO4) = 1.2 x 10^-5
Now we can substitute the known concentrations and solve for the unknown concentration, x:
For PbSO4: (0.012)(x) = 2.0 x 10^-8
For Ag2SO4: (0.048)(x) = 1.2 x 10^-5
Solving for x, we find:
x = (2.0 x 10^-8) / (0.012) for PbSO4
x = (1.2 x 10^-5) / (0.048) for Ag2SO4
Calculating these values, we find:
x ≈ 1.67 x 10^-6 M for PbSO4
x ≈ 2.5 x 10^-5 M for Ag2SO4
Comparing these concentrations, we see that the concentration of Pb2+ in solution (1.67 x 10^-6 M) is smaller than the concentration of Ag+ (0.048 M). Therefore, PbSO4 will precipitate first.
(b) To find the concentration of the first ion precipitated when the second ion starts to precipitate, we look at the equilibrium concentrations. At the point where the second ion just starts to precipitate, the concentrations of both ions will be equal since the solubilities are the same.
Since Ag2SO4 has a higher solubility product constant (Ksp) compared to PbSO4, it will start precipitating first. Therefore, when Ag2SO4 starts to precipitate, the concentration of Ag+ will be equal to the concentration of Pb2+, which is 1.67 x 10^-6 M.