Please help me out. Chem. solubility stuff.

Test Preparation SAT Subject Test Preparation
1

<p>Okay so I was looking at the McGraw Hill's review over solubility rules and I think they did it wrong (page 131, 2006 edition). So tell me which way is right.</p>

<p>Question: Magnesium Hydroxide has a solubility product constant of 1.8 x 10 ^-11 at 298 K. Write the equilibrium constant expression for this salt. What is the concentration of magnesium and hydroxide ions in a saturated solution of magnesium hydroxide?</p>

<p>Equation: Mg(OH)2 = Mg + 2OH</p>

<p>They say: Ksp= ([Mg] [OH]^2)/1</p>

<p>This makes sense, but then they say: 1.8 x 10^-11= [x][x]^2=x^3
Solve for x, x= 2.6 x 10^-4. "Thus [Mg]= 2.6 x 10^-4 and [OH] = 6.8 x 10^-8."</p>

<p>I thought this is how you would do it:
Since there are 2 moles of OH for each mole of Mg, use 2x.
Therefore I thought it'd be 1.8 x 10^-11 = [x][2x]^2= 4x^3
Solve for x, and [Mg]= 1.65 x 10^-4 and [OH]= 3.3 x 10^-4.</p>

<p>Who is right? I taught myself how to do this but I watched videos online and they do it with 2x. Please reply.</p>

2

<p>If I am dumb just tell me! lol I need to know the right way to do it.</p>

3

<p>seriously can somebody please help me?</p>

4

<p>i think you're right..it may be an error</p>

5

<p>thanks for the reply dude. yeah it just didn't make sense to me (their way)
algebraically and analytically</p>

6

<p>eh test tmrw..i actually took it in oct. got a 750 but am tryin to get 800</p>

<p>how are u doing on ur PT's</p>

7

<p>i hate organic chemistry</p>

<p>im doing pretty well on my practice tests...missing around 5-10 questions each </p>

<p>hope the curve is good tomorrow</p>

8

<p>How much Organic Chemistry is usually on the Chemistry tests?</p>

9

<p>actually on the Ksp problem, i think the book was right because you use a mass action law, or whatever its called, equation to find the equilibrium constant--so the coefficients become powers</p>

10

<p>yeah I turned the coefficients into powers in both equations.
but they didn't take into account that for every mole of Mg there are 2 moles of OH, hence [x][2x]^2 not [x][x]^2.</p>

<p>Anyways, in response to diligency, I just took the CB test and got a 770 on it. I hope I can replicate that tomorrow!</p>