okay let's check out a question regarding precipitation reactions this
asks at 25 degrees Celsius the solubility product of mg(oh)2 or
magnesium hydroxide is 1.0 times 10 to the negative 11 at what pH will
magnesium two-plus ions start precipitating in the form of magnesium
hydroxide from a solution of 0.001 molar magnesium ions so go ahead and check out
some of my tutorials regarding solubility and the solubility product if
you need some review and when you're ready to give this a try
okay first things first let's write out the reaction describing the solubility
product that we're given so the solubility product of magnesium
hydroxide that means we're talking about magnesium hydroxide solid dissociating
into magnesium two-plus ions and hydroxide ions so that's this reaction
here or this equilibrium we know that it has very low solubility but we know that
a little bit of this substance is going to dissociate and this must be balanced
so we know we need two hydroxide ions because there are two hydroxides for
every formula unit of the solid now we can also write the KSP expression
because we know that the KSP is equal to 1.0 times 10 to the negative 11 and we
know that that is going to be equal to the magnesium ion concentration times
the hydroxide ion concentration squared because there is a coefficient of 2 in
front of hydroxide in that equilibrium now we know that precipitation is going
to occur when Q is greater than KSP right so we need to have some
information about these magnesium ion and hydroxide ion
concentrations in order to say when precipitation is going to occur so let's
plug in what we know we have KSP we have the magnesium ion concentration in
solution we know that we're point zero zero one molar in magnesium ions so
let's plug that in as well the only thing we don't know is the hydroxide ion
concentration and we will need to solve that in order to say something about pH
so let's solve for the hydroxide ion concentration we just divide both sides
by point zero zero one and then take the square root and we get one point zero
times ten to the negative four so that is the hydroxide ion concentration that
will be required in order to promote precipitation but we don't want the
hydroxide ion concentration we need the pH this is asking for a pH
so first let's get the pOH we know that that means the negative log of 10 to the
negative four which will give us for we know that pH is going to be equal to 14
minus the pOH and that is 14 minus 4 which equals 10 so we know that above a
pH of 10 there will be sufficient hydroxide ions in solution so as to
promote precipitation of magnesium hydroxide.