1. Good News!

MDApplicants login issue has been resolved!

Interview Feedback is now available at: schools.studentdoctor.net

# Kaplan FL 3 PS #42

Discussion in 'Kaplan MCAT Forum' started by PharaohMD, Jul 30, 2008.

1. This forum made possible through the generous support of SDN members, donors and sponsors. Thank you.
1. ### PharaohMDNew Member

Joined:
May 30, 2008
Messages:
6
So I just finished taking number 3 and I am not quite sure of the explanation they give to this questions. Any suggestions?
2. ### futuredoctor10Established Member

Joined:
Jan 25, 2007
Messages:
1,533
Status:
Medical Student
I actually was just looking over Kaplan FL3.

So for this question, I think they made a minor mistake in the explanation (doesn't affect the correct answer). They say "CO2 reacts with H2O to form carbonic acid (HCO3-)." The problem with this: CO2 + H20 makes carbonic acid, but that is H2CO3, not HCO3-. HCO3- is bicarbonate ion!

Again ignoring this error in the explanation, you look at paragraph 2 which states "carbon dioxide will react reversibly with water to form carbonic acid" (CO2 + H20 <--> H2CO3).

Experiment 1 states: The gas... is passed through a solution of BaCl2, producing a white precipitate.

H2CO3 (g) + BaCl2 ---> BaCO3 (s) + HCl
We know HCL can't form the white precipitate, so we have BaCO3 (s). [or solubility rules: carbonates with barium = insoluble]

If alot of the BaCO3 precipitate were lost, then the student would obtain a lower amount of product. Let's say theoretically 10 g of BaCO3 would form but he lost some according to the Q stem, so now let's say he has only 5 g. This means the amount of HCl he calculates (by stoichiometry) will be lower.

A lower amount of HCL = lower amount of H+ = higher pH

Calculated pH is higher than the actual pH

Hope this clarifies their explanation, feel free to post any questions about what I've said. Also anyone pl correct anything stated inaccurately above!