# Decimal points...

Discussion in 'MCAT Study Question Q&A' started by TheJourney, May 12, 2014.

1. ### TheJourney 2+ Year Member

74
7
May 9, 2013
If the Ka of the HClO is 33 x 10^-8, the Kb value of ClO- is:

A. 3 x 10^-7
B. 3.3 x 10^8
C. 3.3 x 10^6
D. 14

Very simple question, but the decimals have me all worked up. I was able to figure this one out knowing that 1x10^-14 = Ka x Kb. All decimal places aside, knowing it must be equivalent to an integer of 1, I was able to reason that 33x3 = 99 which is approx 100 (a factor of 1) and 33x3.3 would be greater than 1. However when I try to solve it, I can't seem to get the right exponent doing it in my head. Kb = (1x10^-14)/(33x10^-8), I re-wrote as (100x10^-12)/(33x10^-8) then dividing I get 3^(-12- -8) = 3^-4, the correct answer choice A which has it to the ^-7. I thought maybe I should have written it as 100x10^-16 and not 100x10^-12, but that would be 3x10^-8, still not 10^-7.

3. ### DrknoSDN

450
104
Feb 21, 2014
The answer is wrong I believe it should be 3 x 10^-8. I Think i remember doing that problem and finding error posted on EK web forums.

Taking the product of the Kb and Ka you would expect to get: 1*10^-14 and not 9.9*10^-14

Czarcasm likes this.
4. ### TheJourney 2+ Year Member

74
7
May 9, 2013
How is it wrong? Wouldn't the error have to be in the exponents.

.33x3 = .99, .99 is close enough to 1, or no?
If I'm mistaken, could you please elaborate.

5. ### DrknoSDN

450
104
Feb 21, 2014
So 9.9 * 10^-14 is almost 1.0 * 10^-13
So it's off by an order of magnitude.

Alternatively use 34 instead of 33.. you would expect to get slightly more than 1x10^-14.
But you don't. You get: 1.02 * 10^-13