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PCAT Chemistry problem (pH and logs)

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PharmaMaya

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Hi everyone,

I'm studying for the august PCAT and I'm having difficulty with the pH and pOH questions. I could do these EASILY during class because we used calculators, but without a calculator I get confused. I've just about figured out that if the molarity is like .1, .01, .001, etc. then I can just count the decimal places. Like .1 would be -1 (so a pH of 1) and .01 would be -2 (pH of 2), etc. Does anyone have any other tips and tricks for working these types of problems easily and quickly?

Thanks :D
 

PharmaMaya

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I think I've found what I was looking for...

Check out this trick I found at http://www.curiousmath.com/modules....le=article&sid=32&mode=thread&order=0&thold=0 (posted below)

1. When someone gives you any positive number, you should immediately 'write' that number in scientific notation in your head.

2. Next, focus only on the exponent of the number (written in scientific notation). This number will be the basis of you answer.

3. Estimate the logarithm of the abscissa in your head (thats the number between 1 and 9.999999..., not part of the exponent). Note: you'll need to memorize the table below (its not that hard).

4. Add the logarithm of the abscissa to the exponent you found in step 2.

What follows are the values for the logs you'll need to have memorized for step 3...

log[1]= 0
log[2]= .30
log[3]= .48
log[4]= .60
log[5]= .70
log[6]= .78
log[7]= .85
log[8]= .90
log[9]= .95

As an example, lets find the logarithm of 29,012. Written in scientific notation, that would be 2.9012 X 10^4. So, the exponent is 4. Now, we need to concentrate on the abscissa (2.9012 is very very close to 3). From our table above (which we have memorized for the trick), the logarithm of 3 is 0.48. So, we add the exponent (4), to the log of the abscissa (0.48), to get a value of 4.48. A calculator reveals how good this method is (4.46 out to two decimal places).

This method works because scientific notation is a base 10 system for writing out numbers. With some practice, you'll get a feel for how to massage your guesses for numbers that aren't exactly in the table (for instance: 2.5).
 

phathead

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From what I remember on the test, the pH and pOH questions are very easy.

They'll give you something like a concentration of H+ ions as 1.0 x 10^-6 and ask for the pH. Naturally the pH is 6.
 
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