[EriLynn~]

so, my midterm is this coming week and i was studying, and i hit a multiple choice example question that looks like this
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.)


BaCrO4 (Ksp = 8.50×10-11)
Ca5(PO4)3F (Ksp = 1.00×10-60)
Ag3PO4 (Ksp = 1.80×10-18)
Hg2Br2 (Ksp = 1.30×10-22)
Pb3(PO4)2 (Ksp = 1.00×10-54)

The answer i got was the order of
2,4,1,3,5 ...

What i did was i found the concentration of the ions, then i multiplied that by molecular weight, then i ordered them.. biggest value to smallest.

finding concentration like this :
(Ca5(PO4)3F) --> (5x)^5(3x)^3(x) = Ksp
84375x^9 = 1.0e-60
x= 6.109e-8 M

next i multiply by molar mass
6.109e-8 x 504.3
= 3.081e-5
then rank them like that..

my values were
22.335e-3
3.081e-5
6.726e-3
9.471e-4
5.042e-9
respective to order of the question ..
what am i doing wrong??

I then tried the whole stoich thing, but apparently i'm doing that wrong too.... i really dont get it .. if you could give me some steps that are more than what i've done that will make sense, that would be amazing!!
could anyone give it a shot, and explain what's going on?
thanks a million folks! lyl =3

[AWK]

Rank the following five salts in order of decreasing solubility

[Borek]

I see how the question can be confusing. It doesn't state how to express solubility - molar? Mass per volume? I would go for molar (which doesn't mean I will get the correct answer - check).

[EriLynn~]

I see how the question can be confusing. It doesn't state how to express solubility - molar? Mass per volume? I would go for molar (which doesn't mean I will get the correct answer - check).

it says mass per unit of volume...

[EriLynn~]

Rank the following five salts in order of decreasing solubility

is that not what i did ??

[AWK]

You showed an order of problems, not the order of increasing solubility. Just sort your results

[EriLynn~]

i'm sorry, i dont understand what you mean by i presented an order of problems ??

[Borek]

I see how the question can be confusing. It doesn't state how to express solubility - molar? Mass per volume? I would go for molar (which doesn't mean I will get the correct answer - check).

it says mass per unit of volume...

Oops, sorry, misread the problem.

But now I don't get you wrote. If I read you correctly you mean that solubility of 2^nd substance, Ca5(PO4)3F, is 3.081e-5 (of whatever). This is lower than solubilities of both 1^st (22.335e-3) and 3^rd (6.726e-3), yet you listed it as the best soluble.

[EriLynn~]

I see how the question can be confusing. It doesn't state how to express solubility - molar? Mass per volume? I would go for molar (which doesn't mean I will get the correct answer - check).

it says mass per unit of volume...

Oops, sorry, misread the problem.

But now I don't get you wrote. If I read you correctly you mean that solubility of 2^nd substance, Ca5(PO4)3F, is 3.081e-5 (of whatever). This is lower than solubilities of both 1^st (22.335e-3) and 3^rd (6.726e-3), yet you listed it as the best soluble.

no no no, i put Ag3PO4 as number one..

[Borek]

Sorry, I have no idea how to make heads and tails of that. How do I know what is what of what? Which of the values listed is silver phosphate solubility and how do I know it was number one when Ag₃PO₄ is third on the list of substances, but list of sorted solubilities starts with number 2?

Please list substances with their calculated solubilities.

[EriLynn~]

i'm sorry it confused you .
the list of the compounds and their Ksp's is the order i'm going with
the numbers are as if you are going down that list putting their rank beside them compound.

BaCrO4 (Ksp = 8.50×10-11)  --- 2nd
Ca5(PO4)3F (Ksp = 1.00×10-60)  ----4th
Ag3PO4 (Ksp = 1.80×10-18) ----- 1st
Hg2Br2 (Ksp = 1.30×10-22)  ----- 3rd
Pb3(PO4)2 (Ksp = 1.00×10-54) --- 5th

is what i had, and its wrong...
make more sense now?

[AWK]

Show your calculations for Hg₂Br₂