Chemistry Q?
2. The Ksp of Mg(OH)2 in water is 1.2(10)^(-11). If the Mg2+ concentration in an acid solution is 1.2(10)^(-5) mol/L, what is the pH at which Mg(OH)2 just begins to precipitate?
Answer:
10N referes to 10 "normal". It means the number of reactive moles per liter in a solution. Similar to molar but not exactly. It equals ~10M for solutions that have an equilibrium which directs all of the ions to dissassociate, which is essentially true for H2SO4, except its a diprotic acid so its normality is double it molarity. http://en.wikipedia.org/wiki/concentrati...
2- you can find out all the formulas you need here
http://en.wikipedia.org/wiki/solubility_...
just plug in your concentrations and work the numbers, should'nt be hard but I'm not doing it for you.
10 N H2SO4 means 10 Normal. For H2SO4, a diprotic acid, this means 5 Molar (2 protons / mole) or 5 Moles H2SO4 per liter.
pH = 14 - pOH
pOH = - log {1.25x10^-11/[Mg2+]} =
-log [1.25x10^-11/1.2x10^5}
1) Not really enough to answer the question. Normality (N) is like molarity (M, moles/liter) but with equation ratios figured in. The trouble is, you have to specify what reaction you are interested in to get the normality. I suspect you are interested in having H2SO4 dissociate into 2 H+ ions, so 10 N = 5 M H2SO4.
2) The Ksp (solubility product constant) of Mg(OH)2 is 1.2 x 10^-11. That means that when the product Mg*OH*OH>Ksp, the product will precipitate. Notice you have to multiply OH concentration twice because there are 2 OH's in the formula of the undissociated compound.
So, 1.2x10^-11 = Mg * (OH)^2
(OH)^2 = 1.2x10^-11 / 1.2x10^-5 = 1.0 x 10^-6
(OH) = 1.0 x 10^-3
p(OH) = -log(OH) = 3 .0
so pH = 14 - p(OH) = 11.0
It means, if you can tip the pH of solution past 11, Mg(OH)2 will just begin to precipitate.
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