Acid base question...Logarithum estimation

[Smitha7]

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So this is a Question out of a Kaplan book. What is the approximate concentration of H+ in normal rain due to the reaction between CO2 and H20? You are given that the pH of rain is= 5.2. The answer choices are:

A. 8x10-3 M
b. 6x10-5 M
c. 7x10-6 M
d. 2x10-7M

How would you go about doing this problem without a calculator?

 

[indygobu]

You will have to know how to compute this backwards and forwards for when you take the MCAT. pH is a function of [H+] (-log(H+) to be exact). The best way, in my mind, to do this quickly is to realize that -log(10^-x) = x. Now, you need to find which [H+] = pH of 5.2. I would first attack this problem by realizing that 5.2 is almost 5. So, you will need an answer that is close to 10^-5 M. This eliminates A and D as possible answers because they are at least 2 orders of magnitude off of what you need. Now you have B and C as possible answer choices. 6 x 10^-5 M will give you a pH that is greater than 5 but less than 6. 7 x 10^-6M will give you a pH that is between 6 and 7. So by Process of Elimination, the answer should be B. My advice would be to find a gen chem book, if you need more practice on this, and go to the pH section and find problems that require you to compute pH from a concentration and a concentration from pH, don't use your calculator but guess, and check the solutions manual to your guess. Hopefully you get better at this because this is an iconic question that would be asked on the real MCAT.

 

[MilkIsGood]

You will have to know how to compute this backwards and forwards for when you take the MCAT. pH is a function of [H+] (-log(H+) to be exact). The best way, in my mind, to do this quickly is to realize that -log(10^-x) = x. Now, you need to find which [H+] = pH of 5.2. I would first attack this problem by realizing that 5.2 is almost 5. So, you will need an answer that is close to 10^-5 M. This eliminates A and D as possible answers because they are at least 2 orders of magnitude off of what you need. Now you have B and C as possible answer choices. 6 x 10^-5 M will give you a pH that is greater than 5 but less than 6. 7 x 10^-6M will give you a pH that is between 6 and 7. So by Process of Elimination, the answer should be B. My advice would be to find a gen chem book, if you need more practice on this, and go to the pH section and find problems that require you to compute pH from a concentration and a concentration from pH, don't use your calculator but guess, and check the solutions manual to your guess. Hopefully you get better at this because this is an iconic question that would be asked on the real MCAT.

i think it would be C, not B. your pH ranges are wrong

 

[Smitha7]

Your right the answer is c

 

[indygobu]

Hahaha, oops. That's what I get for not waking up prior to doing MCAT math. Use the same logic I used in my original post. 6 x 10^-5 is closer to 1 x 10^-4 so it would have a pH range from 4-5. C which IS the correct answer (sorry): 7 x 10^-6 is closer to 1 x 10^-5 so it would have a pH range from 5-6. Again, sorry.