NS Chem&Phy Strategy + Practice Section 2 #5! Help!

[junemcat]

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This question is from Passage 2 #5.

What is the "saponification number" for the fat shown in Figure 1?
a. 46
b. 66
c. 140
d. 197

In figure 1, the formula of this triglyceride is C55H98O6 (MW = 854 g/mol).

In the passage, it is stated that:
"saponification number" = "the number of mg of KOH required to completely hydrolyze 1g of fat with a particular average molar mass"

I am not understanding what this question is asking for?

Sorry if this is vague, thanks in advance!

 

[3AA]

Do they give any relationship between KOH and the fat (any stochiometric relationship?).

 

[JustinM88]

This question is from Passage 2 #5.

What is the "saponification number" for the fat shown in Figure 1?
a. 46
b. 66
c. 140
d. 197

In figure 1, the formula of this triglyceride is C55H98O6 (MW = 854 g/mol).

In the passage, it is stated that:
"saponification number" = "the number of mg of KOH required to completely hydrolyze 1g of fat with a particular average molar mass"

I am not understanding what this question is asking for?

Sorry if this is vague, thanks in advance!

Ultimately you’re looking for mgKOH required to break apart (hydrolyze) those esters within that triglyceride (freeing the fatty acids from the glycerol backbone).

I think the mol ratio would be 3molKOH to 1 molTriglyceride because there are 3 esters in a triglyceride and it takes a mole of OH- to hydrolyze each of them.

I’m thinking:
1g Triglyceride * 1molTriglyceride/854gTryglyceride * 3molKOH/1molTriglyceride * 56gKOH/1molKOH * 1000mg/1g = 197mgKOH

I got 197mgKOH, what’s the answer?

 

[NextStepTutor_1]

Hi all -

The correct answer is indeed 197 (corresponding to 197 mg KOH), although depending on the steps you take to simplify the math through rounding, you might get an answer more like 200 that you have to round down.

You're not supposed to have heard of the term "saponification number" before -- the idea is to introduce a new term in the passage and then ask you to apply stoichiometry using your knowledge of the structure of triglycerides and how esterification works (keeping in mind that saponification as a reaction is something that you should know of) + dimensional analysis to work out the conversion.

Hope this (as well as the very helpful answers above!) makes sense, & best of luck in your review!

 

[junemcat]

Thanks all for the response!! Appreciate it!