Molar solubility....

sellingthings101

So, what exactly is the reason for exponents in the Ksp expressions?
Let me show you why I'd like to know!

Consider calcium phosphate. What is the calcium concentration if the Ksp= 2 x 10^-30

Well,
Ksp= [Ca]^3 [PO4]^2
2 x 10^-30 = (3x)^3(3x)^2

Once I solve for x why do I not also cube the answer to get the concentration, in addition to multiplying by 3?

Really.. what is the reason for the exponents?

WheatLom

2+ Year Member
Equation 1

A <-> B

A = 5 B = 10 K= /[A] K=2

double the concentrations.
2A <->2B

we square the concentrations in the K expression. K = B^2 / A^2 K=100/25 which equals 4.

K=4 and that seems different than the previous reaction when K=2 but you did double everything from the first reaction. If you take the square root of the 2nd reaction you get K=2. Taking the root we get the original K value. for 1 mole of A and B.

So the rates are the same, but the changes in moles are different due to the different concentrations you start with.

If you don't square it you get 20/10 K=2 but.... that doesn't take into account that more moles are in the reaction. Obviously you would be making different amounts if you had double the recipe vs. a single serving of it.

I hope this helps.

Last edited:
LuckBloodandSweat