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Author Topic: Determing whether a mixture will form a precipitate from Ksp calculation  (Read 224 times)

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PopindaChopz98

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Consider solutions A and B:

Solution A: 0.010 mol/L Pb2+(aq)
Solution B: HBr(aq) with pH = 1.52

Determine whether or not a precipitate of PbBr2(s) (Ksp = 4.7 x 10-6) would form when a sample of solution A is mixed with a sample of solution B which has exactly twice the volume of the solution A sample. Show calculations to justify your answer.

Could someone help me out with this please? I have no idea how to answer this (specifically the volume part).
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Corribus

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First I would write a balanced equation and define what Ksp means in terms of it.
If the relative volumes are throwing you off, just pick specific values (e.g., 1 L of volume A and 2 L of volume B).
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PopindaChopz98

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I assumed that the way I would do it was simple. I started by writing the ionization equation for PbBr2(s) and I got that Ksp=[Pb2+][Br-]2 but after that. I'm stumped. I was also thinking maybe I would just say that since HBr is a strong acid it completely ionizes and therefore the concentration of H3O+ is equal to the concentration of Br- but where does the "exactly twice" volume come in there? :/ maybe is it meant to throw me off??
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Corribus

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You are on the right track with the acid - it is fully dissociated so if you know the pH, you should know the
 concentration in solution B.
You also obviously know what the [Pb] is in solution A.
The volumes come into play in that when you mix the two solutions, the concentrations of each species in the mixture will change compared to what they are in the isolated solutions before mixing.

So, let's assume you take 1 L of solution A and 2 L of solution B, and mix them. We can figure out the concentrations of each ion is in the new mixed solution by calculating the number of moles of each ion in each solution (which we can do because we know the volumes), then divide by the new volume in the mixture to determine the final concentrations. With this, you should be able to apply the Ksp condition to determine if there is a precipitate.

Note: you will get the same answer if you put in 2 L and 4 L, or 4 L and 8 L... or generalize the relative concentrations. But putting in specific values can make solving the problem a little more straightforward.
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