[VanKwisH]

Rank the following five salts in order of decreasing solubility, in terms of mass per unit volume.
(The most soluble gets rank 1, the least soluble gets rank 5.)


Hg2Br2     (Ksp = 1.30×10-22)
Ca3(PO4)2     (Ksp = 1.30×10-32)
Ca5(PO4)3OH     (Ksp = 6.80×10-37)
Hg2CO3     (Ksp = 9.00×10-15)
Ag3PO4     (Ksp = 1.80×10-18)

What I did was, I created an Initial,Change,Equilibrium Table. From that I was able to
derive an expression in terms of x for each corresponding Ksp value. Then I solved for x ( units=mol/L ).
And I converted the answer to g/L.

My work:
Hg2Br2 - Using an ICE table i came with the following expression
(2x)^2 * (2x)^2 = 1.30*10^-22
16x^4 = 1.30*10^-22

Solving for X gives me 1.6883*10^-6 ( mol/L)
converting gives me 9.47*10^-4 g/L

My numbers for the other compounds are
Hg2Br2     9.47*10^-4 g/L
Ca3(PO4)2     5.102*10-5 g/L
Ca5(PO4)3OH     1.365*10^-2 g/L
Hg2CO3     6.043*10^-3 g/L
Ag3PO4   6.264*10^-3 g/L

According to the question my ranking is. 4,5,1,2,3
But this is wrong. Can anyone help out with this ?

[Jorriss]

All you need are the KSP values themselves. No ice table necessary.

[VanKwisH]

I read in my book, that a lower KSP value makes the compound more soluble.
Therefore that would make the order
3,2,1,5,4

BUT that ranking is wrong.. so what should i do ?

[Jorriss]

I read in my book, that a lower KSP value makes the compound more soluble.
Therefore that would make the order
3,2,1,5,4

BUT that ranking is wrong.. so what should i do ?

You're book 'may' have meant lower in the sense that the exponents absolute value is lower?

But, let's think it through.

Let's use the last example, Ag3PO4.

So we have the equilibrium, Ag3PO4 <--> 3Ag+ + PO43-

The Ksp is then going to be: Ksp = [Ag+]^3[PO43-]



This means, the larger the Ksp, the more soluble the product as there is more of the product present !



The answers you gave are in the exact opposite order.

[VanKwisH]

OOK, BUT then the order would then be
3,4,5,1,2

I've tried that, but that also is not correct . AH im so lost!

[Jorriss]

OOK, BUT then the order would then be
3,4,5,1,2

I've tried that, but that also is not correct . AH im so lost!

Remember, it's based on these Ksp's.
Hg2Br2     (Ksp = 1.30×10-22)
Ca3(PO4)2     (Ksp = 1.30×10-32)
Ca5(PO4)3OH     (Ksp = 6.80×10-37)
Hg2CO3     (Ksp = 9.00×10-15)
Ag3PO4     (Ksp = 1.80×10-18)

[Borek]

Ag3PO4     (Ksp = 1.80×10-18)

Ag3PO4   6.264*10^-3 g/L

Show - step by step - how you got from the first number to the second.

I read in my book, that a lower KSP value makes the compound more soluble.

As it was already signalled, quite the opposite. However, you are not interested in just a molar solubility, but in mass solubility - so you need to take also molar masses into account. And remember, that Ksp depends on the number of ions produced, so it can't be directly compared for a very different compounds.

[Jorriss]

Ag3PO4     (Ksp = 1.80×10-18)
 but in mass solubility - so you need to take also molar masses into account.

Man, I missed that. I have been extremely failing lately.