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Topic: minimum number of moles of KOH required to saponify 13.7 grams  (Read 6914 times)

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Offline billsauce

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minimum number of moles of KOH required to saponify 13.7 grams
« on: April 13, 2013, 03:20:02 PM »
i dont know how to start this problem. do I need a balanced equation then solve for the number of moles of KOH?
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Offline Arkcon

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #1 on: April 13, 2013, 03:31:53 PM »
This is a pretty straightforward reaction.  You should have a balanced equation, and be able to use it to determine how much NaOH you need.  You will also have to be able to convert grams to moles, in each case.
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Offline billsauce

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #2 on: April 13, 2013, 03:45:38 PM »
you mean how much KOH I need?

i figured it should just be 13.7g/136.15g (molar mass) = .101 moles, and since its a 1 to 1 ratio its .101 moles KOH
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Offline billsauce

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #3 on: April 13, 2013, 04:27:41 PM »
or does saponifcation require 3 moles, so it would be .303 mol KOH?
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Offline Arkcon

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #4 on: April 13, 2013, 06:03:26 PM »
Write a balanced equation and see how many moles are needed.  To save yourself some drawing, break the substrate into two groups, each part the part that reacts with part of the NaOH.  For example: neutralization of citric acid:

[Citrate]H3 + 3 NaOH  :rarrow: [Citrate]Na3 + 3 H2O
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Offline billsauce

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #5 on: April 13, 2013, 06:21:46 PM »
ok so ti would be 3 moles of KOH needed, meaning .300 moles of KOH correct?
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Offline Arkcon

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #6 on: April 13, 2013, 08:26:44 PM »
No.  Actually balance your equation and show us the work that leads to your answer.
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Offline billsauce

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #7 on: April 13, 2013, 08:39:29 PM »
C8H8O2 + KOH ---> C7H5OK + CH3OH


so a 1 to 1 mole ratio?? which means .100mols of KOH?
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Offline Arkcon

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #8 on: April 13, 2013, 09:18:14 PM »
Yes.  Now, how many moles of methyl benzoate do you have?
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Offline billsauce

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #9 on: April 14, 2013, 12:46:40 AM »
13.7g of C8H8O2/ 136.15g/mol(molar mass of meythl benzoate) is .101 mol, and since its a 1 to 1 mole ratio, it requires .101 mole KOH for saponification?
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Offline billsauce

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #10 on: April 14, 2013, 12:45:55 PM »
?
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Offline sjb

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #11 on: April 14, 2013, 01:40:38 PM »
Quote from: billsauce on April 14, 2013, 12:46:40 AM
13.7g of C8H8O2/ 136.15g/mol(molar mass of meythl benzoate) is .101 mol, and since its a 1 to 1 mole ratio, it requires .101 mole KOH for saponification?

Sounds good.
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Offline Arkcon

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #12 on: April 14, 2013, 02:40:45 PM »
Now, compute how many grams of NaOH that is.  I know this question doesn't ask that, but the question on the exam might.  You want to be able to do this sort of problem forward and backward so you can do them quickly.
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Offline iCreateOxytocin

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Re: minimum number of moles of KOH required to saponify 13.7 grams
« Reply #13 on: April 14, 2016, 10:05:37 PM »
Hi, I have the same exact question just with a different desired final number of moles. I really appreciated following the stream. I would like to know how to take the problem the next step further as you suggested, as I can see my professor very much wanting us to go the next step :).

My question:

What is the minimum number of moles of potassium hydroxide (KOH) required to
saponify 20.2 g of methyl benzoate?

20.2 C8H8O2/136.1C8H8O2=0.148

Is this correct? how do I take it a step further?
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