question with solubility/precipitates

[yoyohomieg5432]

I'm really trying to grasp the overall concept with these types of problems. This question was from AAMC test 5: what was most likely the identity of the precipitate formed when NaCl was added to Cd(NO3)2?

is Cd(NO3)2 considered a complex ion? so it exists in a solid form, and when placed in water it can be broken up into Cd2+ and 2NO3- . In the solution is also Na+ and Cl-. I can then imagine that Cd2+ and Cl- interact to form CdCl2.

There is NO way to tell whether this actually precipitates, right? It's just hypothetically speaking if a precipitate were to occur it would be CdCl2? To actually determine whether a precipitation occurs we would have to know the concentration of Cd and Cl to calculate the Q and then compare it to Ksp for CdCl2 and from that we could tell whether or not it precipitates, right?

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[sazerac]

An important MCAT skill is to read the answer choices as well as the passage and the question stem.

Obviously, given those answer choices, they are asking you for the charge-balanced formula of cadmium chloride. You figure that Cd is 2+, you know that Cl is -, you answer the question, and you move on.

Just answer the question and move on. Worrying about concentrations and Q values and Ksp on a question like this is a good way to waste time and get a lower score.

 

[GomerPyle]

I'm really trying to grasp the overall concept with these types of problems. This question was from AAMC test 5: what was most likely the identity of the precipitate formed when NaCl was added to Cd(NO3)2?

is Cd(NO3)2 considered a complex ion? so it exists in a solid form, and when placed in water it can be broken up into Cd2+ and 2NO3- . In the solution is also Na+ and Cl-. I can then imagine that Cd2+ and Cl- interact to form CdCl2.

There is NO way to tell whether this actually precipitates, right? It's just hypothetically speaking if a precipitate were to occur it would be CdCl2? To actually determine whether a precipitation occurs we would have to know the concentration of Cd and Cl to calculate the Q and then compare it to Ksp for CdCl2 and from that we could tell whether or not it precipitates, right?

I believe you are correct - you need to know the concentrations and solve for Q, and if it is less than Ksp, than it hasn't precipitated yet.