gen chem problem

[topdent1]

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[SIZE=+1]The Ksp for PbCl2 is 1.6 * 10–5. Which maximum amount of NaCl(s) may be added to 1.0 L of 0.010 Pb(NO3)2 solution without causing precipitate to occur?[/SIZE]
[SIZE=+1] A. 1.6 * 10-5 mol[/SIZE]
[SIZE=+1] B. 3.2 * 10-5 mol[/SIZE]
[SIZE=+1] C. 1.6 * 10-3 mol[/SIZE]
[SIZE=+1] D. 3.2 * 10-3 mol[/SIZE]
[SIZE=+1] E. 1.6 * 10-2 mol

Can someone give me a detailed solution please?
[/SIZE]

 

[harrygt]

Oooooooooooo ok I think I see it. I carefully read through your explanations. So, since PbCl2 is not actually dissociating itself. You have Pb(NO3) with a given concentration, and they are telling you that you are going to add NaCl as the source of Cl-. So when you write the equation......you have [Pb] (given) and [Cl-] from NaCl that you are "going to put in".......ok. I hope that made sense. I think thats what it is....and thats why you are not having [2Cl]. But if thats the case.....you still have to square it??

It's good that you are capturing it now. Yes, you still have to square. That is the definition of Ksp, no matter what!

 

[Danny289]

harrygt

check you email 😉

 

[Mstoothlady2012]

You have to square Cl conc, because that is how you find the ion product [product of each ion conc that has been reaised to the coefficient of the balanced reaction.]
You have to put the concentration of each ion in its bracket. if we had x moles of PbCl2 dissovling in the solution, we would have 2x Cl-, and therefore, we would put 2x in the brackets of Cl- and raise it to 2. For this case, the reasaon is simply obvious. You get 2x Cl- for each x mole disssociated PbCl2, so 2x is the Cl- conc.

this question is different though. There is no x moles of PbCl2 dissolving to give us 2x moles of Cl-. As mentioned in the prev paragraph, you should put the real conc of Cl- in the brackets. if we add x moles of Cl-, how many moles of Cl- would there be in the solution? Of course X!
Look at it this way. What you are doing [the wrong way] is that you put x Cl- in the solution, but then you put 2x in the brackets. This means based on your way of solving the problem, the Cl- conc is getting doubled somehow. You are adding X mole, and then declaring the conc of the Cl- to be 2X. This is not right.

Like you said, if we add x mol of Cl we should get x mol of Cl not x^2

 

[harrygt]

Like you said, if we add x mol of Cl we should get x mol of Cl not x^2

Lol, yeah, we get x moles of Cl, but the Ksp formual does not change.😀 x^2 is not the same as the CL- conc. It's just the the way Ksp fo PbCl2 is defined

 

[DentalP87]

I agree with Harry's answer. He is 100% correct.