# Ksp

#### Hemichordate

##### Peds
10+ Year Member
If you have a 0.0001 M solution of HCl, why do you get 0.0001 mol H+ and 0.0001 mol Cl-?

If you use the Ksp = [H+][Cl-] = 0.0001, you would get [H+] = 0.01 mol.

Why is this not the case?

#### Will Hunting

##### Aspiring Cardiologist
10+ Year Member
5+ Year Member
If you have a 0.0001 M solution of HCl, why do you get 0.0001 mol H+ and 0.0001 mol Cl-?

If you use the Ksp = [H+][Cl-] = 0.0001, you would get [H+] = 0.01 mol.

Why is this not the case?
That is the maximum. Remember that. The ksp is used to solve for the molar solubility. If the concentration is less, then that amount is dissolved. .01 is greater than .0001. If the molar solubility were 10 M/L and I have 1 M/L then all of it will dissolve and the amount of moles is 1 mole assuming one liter. However, if I have 100M/L. Then, only 10 Moles will dissolve and 90 moles won't. HTH.

Your case is the same, the max is .01 but you have less. You're confused because the vast majority of the time the M/L is greater than the molar solubility so what dissolves is the molar solubility. This is only if the solution is saturated.

OP
H

#### Hemichordate

##### Peds
10+ Year Member
Thanks will. So basically with strong acids/bases, it dissociates completely, so you don't need to know the Ksp (just know the molarity), but with weak acids/bases, you only get partial dissociation, so you need to know the Ksp.
So whether or not you use the Ksp value depends on what compound you have, right?

#### wanderer

10+ Year Member
5+ Year Member
Thanks will. So basically with strong acids/bases, it dissociates completely, so you don't need to know the Ksp (just know the molarity), but with weak acids/bases, you only get partial dissociation, so you need to know the Ksp.
So whether or not you use the Ksp value depends on what compound you have, right?
I think you are confusing Ksp and Ka. You can have a strong base that isn't fully soluble (I think Mg(OH)2 is one such example).

OP
H

#### Hemichordate

##### Peds
10+ Year Member
Ok nvm, I am confusing myself.

#### OMTlover

I think you are confusing Ksp and Ka. You can have a strong base that isn't fully soluble (I think Mg(OH)2 is one such example).
This is absolutely right. Ksp refers to molar solubility when solute is dissolved in solvent. The concept of Ksp is usually applicable to salts. Whenever the ion product is > than its Ksp, the percipitate will form. Mg(OH)2 has a pretty low Ksp value. Meaning if Q>Ksp, the percipitate will form.