Here you go:
http://forums.studentdoctor.net/threads/berkeley-reviews-unique-approach.980939/
the log trick has really helped me:
Log Math Trick
The Know Your Primes Method
First and foremost, you should always look at the answer choices and see how much precision you need. If the answers are far apart, then you can afford to make less rigorous approximations. But if they are close to one another, as they could be from time-to-time, then precision is necessary.
For determining pH from [H+], or any other conversion that involves taking a negative log, we use the following relationship.
- log (a x 10-b) = b - log a
This is applicable for pH, pOH, pKa, and pKb.
Next we teach the
know your primes approach. Know the following four logs for approximating a best answer:
log
2 = 0.30
log
3 = 0.48
log
5 = 0.70
log
7 = 0.85
Because prime numbers can be multiplied together to get other numbers, if you need precision you can build from those numbers. And the prime numbers between 1 and 10 will give you the necessary precision to make a good choice on 99.9999999% of the MCAT questions you'll see.
Given Ka = 4.61 x 10-7; pKa = 7 - log 4.61 which is slightly larger than 7 - log 5 = 6.3. So guessing around 6.33 +/- is going to be as much precision as you could need on the MCAT.
Given [OH-] = 2.77 x 10-4; pOH = 4 - log 2.77 which is slightly larger than 4 - log 3 = 3.52 but not as large as 4 - log 2 = 3.7. So guessing around 3.56 +/- is closer than you will likely need.
Given [H+] = 7.93 x 10-3; pH = 3 - log 7.93 which is slightly smaller than 3 - log 7 = 2.15. So guessing around 2.11 +/- is good enough. This is where the proponents of precision will say that knowng 3 - log 8 = 2.10 gets you a more accurate answer. And I can't deny that 2.10 is closer to 2.097 than 2.11, but if the MCAT choices are so close that 2.10 beats 2.11, then the test would have changed so much you would have heard someone complain about log details.
Given Kb = 6.11 x 10-8; pKa = 8 - log 6.11 which is larger than 8 - log 7 = 7.15, but less than 8 - log 5 = 7.3. So guessing around 7.23 +/- is a winning approximation.
Picking the method that works for
you is important, because you have to balance the need for speed with your level of satisfaction with an answer before you can move on without lingering second thoughts. The
know your primes approach is a great method to find that balance.
