Solution PH question.

[Jonbrout]

It has been several years since i have taken chem and I cant seem to find any information that helps me solve this problem.

What is the ph of a 1.0x10-4M Ca(OH)2 solution.

I realize you will get 2 OH's from the dissociation of the Ca(OH)2 but then you will have to take the -log.

Like this: [OH]= 2 x (1.0x10-4) = 2.0 x 10-4
and THEN [pOH] = -log (2.0x10-4)

Im just confused about the freaking -log part. No calculators of course. Could someone enlighten my ignorance. I simply don't remember how to do -logs in my head.

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[Ismet]

You'll have to do a review of logarithms. You can ballpark it in your head or on scratch paper and find the answer that is closest to your estimate.

We know that
log(1e4)=4.

So log(1e-4)=-4.

Then, -log(1e-4)=4.

-log(2e-4) will be slightly less than 4.
Putting it into a calculator, the answer is 3.699.

 

[kehlsh]

-log (1.0x10-4) = 4
-log (1.0x10-3) = 3
-log (2.0x10-4) is in between so.. it will be 3.X

I don't think mcat will give you multiple 3.x's

 

[Swagster]

I think we all forget logs. They come back with practice. Berkeley Review showed a cool trick in their book where -log (2.0x10-4) is 4 - log 2 = 4 - 0.3 = 3.7. The pH would be 4-3.7 or 10.3. You can memorize that log 2 = 0.3 or look at the answer choices and figure that log 2 is small, so the pOH will be barely less than 4 and the pH barely more than 10.

 

[SN2ed]

Thread moved. This type of question belongs in the Study Q&A forum.

 

[Cosmic]

Think of it like this. The negative log (or the 'p' term in pH) is a convenient way to get rid of the negative sign in the exponent. That's actually why it's used because concentrations of hydrogen or hydroxide ions are usually very small (on the order of 10^-#). If they instead used regular log values it would of been -#. Writing negative signs would be very redundant, so that's why we use the 'p' or negative log instead.

Basically the pH is the # part of 10^-# unless of course there's a number in front other than 1. If it were 10^# it would be -# because there's no negative sign.