MCAT Chemistry and Physics Strategy & Practice

ArielM
Posts: 43
Joined: Mon Oct 02, 2017 9:15 pm

MCAT Chemistry and Physics Strategy & Practice

Postby ArielM » Wed Dec 20, 2017 5:43 pm

My question is found in the Chemistry Physics S&P Book, Timed Section 2, Passage 2, Question 5.

Question 5 states:
What is the "saponification number' for the fat shown in Figure 1?

The answer choices are:
A. 46
B. 66
C. 140
D. 197
The answer is D, 197.

It is stated at the end of Paragraph 1 that, "Soap makers often will quote a "saponification number", which represents the number of milligrams of potassium hydroxide required to completely hydrolyze one gram of that fat with a particular average molar mass."

Figure 1 depicts an example of a triglyceride with a MW of 854 gmol^-1.
(Attached is the photo of the Triglyceride Molecule)

How do we know that there are 3 moles of KOH for every one mole of fat? Is it because there are three fatty acid esters- therefore, KOH will hydrolyze or cut at the ester bond of each of the three fatty acid esters, for every one mole of fat? How am I supposed to know that there are 3 moles of KOH for every one mole of fat? I'm sure that there is a plain and simple way to understand the answer to this question- I just don't know the way. Please help.

Sincerely,
-Ariel Morrow
Attachments
Photo on 12-20-17 at 4.41 PM.jpg
(103.22 KiB) Not downloaded yet
NS_Tutor_Andrew
Site Admin
Posts: 512
Joined: Mon May 23, 2016 1:47 pm

Re: MCAT Chemistry and Physics Strategy & Practice

Postby NS_Tutor_Andrew » Wed Dec 20, 2017 6:48 pm

Hi Ariel,

Sounds like you've got this! The insight is precisely that one mole of KOH will be needed for each fatty acid chain in the triglyceride, for an overall stoichiometry of 3 moles of KOH per mole of triglyceride. The overall structure of triglycerides is absolutely something you should be quite familiar with, so the Q is expecting you to draw on that as a piece of outside info.

Hope this helps!
Andrew D.
Content Manager, Next Step Test Prep.

Return to “Post your questions here about Next Step's MCAT books”