# Calculating Ksp

#### goodie

7+ Year Member
The whole concept of calculation Ksp and such is really confusing me. Im working through the destoyer and in some questions you only use the coefficient of the reaction as exponents in others you use them as exponents and multiply the whole expression by it. I know this is unclear let me give an example`

Q: Consider Calcium Phosphate, what is the calcium concentration if the Ksp = 2x10-30

the answer is 3^root3 of (2x106-30/108) which means they did

Ca3(PO4)2--> 3Ca + 2PO4

ksp= [3Ca]^3 [2PO4]^2.. why is it this expression and not ksp= [Ca]^3 [PO4]^2

Ive seen discrepancies like this in the common ion effect problems too. Im so confusedd!!! Thanks for any help!!! #### JOoa0ky

5+ Year Member
It is not a discrepancy when every solubility problem is solved like that lol.

Sorry... I don't remember the exact reason why you do it like that but that is how its always been done even in gen chem II.

#### UnimaasMED

10+ Year Member
5+ Year Member
The whole concept of calculation Ksp and such is really confusing me. Im working through the destoyer and in some questions you only use the coefficient of the reaction as exponents in others you use them as exponents and multiply the whole expression by it. I know this is unclear let me give an example`

Q: Consider Calcium Phosphate, what is the calcium concentration if the Ksp = 2x10-30

the answer is 3^root3 of (2x106-30/108) which means they did

Ca3(PO4)2--> 3Ca + 2PO4

ksp= [3Ca]^3 [2PO4]^2.. why is it this expression and not ksp= [Ca]^3 [PO4]^2

Ive seen discrepancies like this in the common ion effect problems too. Im so confusedd!!! Thanks for any help!!! Try to look at it this way. It might be less confusing. Whenever they ask you about the ksp equation of a reaction you only use the coefficient as exponent. E.g. what is the ksp equation of the following reaction:
Ca3(PO4)2--> 3Ca + 2PO4
Here the answer is: ksp= [Ca]^3 [PO4]^2

However, in the problems where you are given ksp and you are asked to solve for concentration of an ion you have to multiply it by the coefficient as well.
For the above reaction, Ca3(PO4)2 will dissolve in Ca^2+ and PO4^3+ in 3 to 2 ratio.
Therefore [3x]^3 [2x]^2= 2x10-30

I hope that helps!