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Topic: Grapefruit Acidity  (Read 8448 times)

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indi_girl18

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Grapefruit Acidity
« on: March 29, 2005, 08:51:09 PM »
First of all, I am new at this whole forum thing, so hopefully I am doing this right.

Second of all, I am doing this research project for my homeschool chemistry course, and I am having a difficult time determining how this certain chemical equations reacts.

The chemical equation begins with C6H8O7 + NaOH.  Now I have tried determining how this reacts, however, I always end up either making up a chemical that doesn't exist or I either don't have enough of a certain element or there is an element left out.

What I have so far is C6H8O7 + NaOH -> C6H6 + H2O + NaOH.  I know this isn't correct because is shows that my base did not react. The second thing that is wrong is that I do not have enough O's in the chemical equation.

Please tell me what I am doing wrong. I need some help. I hope I gave all the necessary information needed for help.

Thank you

Offline hmx9123

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Re:Grapefruit Acidity
« Reply #1 on: March 31, 2005, 02:05:49 AM »
Sorry folks, I totally messed up the splitting of this post from another.  Below are what follows the replies to this topic.
  --hmx9123

Mitch:  What do bases like to do?

indi_girl18: Well, in my experiment, I am trying to determine the acidity of a grapefruit.  So in my titration, I have to add an indicator.  My indicator is phenolphtheleln solution.  In the experiment, after adding my indicator, the base will change the color of my solution thus helping me determine the acidity of my grapefruit.

AWK:  Citric acid (tricarboxylic acid) has formula C6H8O7. It can react with up to 3 molecules of NaOH.

indi_girl18:  I'm confused when you say it can react up to three molecules of NaOH.  What do you mean?

Mitch:  What do bases like to do?
« Last Edit: March 31, 2005, 02:53:29 AM by hmx9123 »

Offline AWK

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Re:Grapefruit Acidity
« Reply #2 on: March 31, 2005, 02:32:26 AM »
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« Last Edit: March 31, 2005, 02:33:15 AM by AWK »
AWK

Offline hmx9123

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Re:Grapefruit Acidity
« Reply #3 on: April 01, 2005, 06:33:16 AM »
AWK does a good job showing citric acid in his picture.  The reason you have to look at the structure is because C6H8O7 isn't very descriptive of the molecule.  You have to know how many acidic protons you're looking at, and in the case of citric acid, there are three acidic protons.  If you want to know how to identify them based on looking at the molecule, look under 'carboxylic acids' in an organic chemistry textbook.  They are the COOH groups on the molecule.  

Remembering that bases react with acidic protons to form water is a key step in figuring this out.  The idea is that for each acidic proton, you'll need one molecule of OH- to neutralize it.  So, as AWK is saying, you have to use 3 mols of NaOH for each mol of citric acid you want to neutralize.  Hope this helps.

Offline Borek

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Re:Grapefruit Acidity
« Reply #4 on: April 02, 2005, 04:34:14 AM »
While this is not necessarilly the best way to describe compounds, sometimes it is easier to understand what is going on if you use more descriptive way of writing down the formula.

Think of citric acid as of (C3H2OH)(COOH)3 - this way you see all three carboxylic groups.

The problem is, it is not possible to prepare such formulas in any systematic manner so they can't be used for general purposes. But they can be pretty usefull.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

indi_girl18

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Re:Grapefruit Acidity
« Reply #5 on: April 05, 2005, 12:43:56 PM »
Ok, thank you all very much. :)  Needless to say, I feel a bit over my head right now, but I think I am beginning to understand what you are saying.  

One thing I do understand is that in order to determine the right equation for my experiment, I have to use the Acid + Base -> Salt + Water equation.  What my teacher said, is that I have to determine how many H's will be given off from the Citric Acid in order to form water.  Now, I havn't determined this yet, though I think I am getting close.  

Again, thank you all very much.  I will post again if I have any further questions, which undoubtably I will. :)

indi_girl18

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Re:Grapefruit Acidity
« Reply #6 on: April 05, 2005, 06:05:33 PM »
Ok like I said, undoubtably I would have more questions, and I do.  How can I tell how many Na's I will be using?

Offline hmx9123

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Re:Grapefruit Acidity
« Reply #7 on: April 06, 2005, 04:01:54 AM »
NaOH comes as a package.  For every mol of NaOH you use, you'll have one mol of sodium ions.

indi_girl18

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Re:Grapefruit Acidity
« Reply #8 on: April 12, 2005, 10:44:42 PM »
Oh, ok. I think I understand.

I have just one more question.  I don't know if anyone can tell me this, but does anyone know how I can determine the amount of citric acid in a grapefruit?

If anyone can tell me this, thank you so much, if no one can, thanks anyways.

Offline hmx9123

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Re:Grapefruit Acidity
« Reply #9 on: April 15, 2005, 03:05:34 AM »
I'm sure there's fairly easy ways to titrate citric acid itself, but the problem is that you've got other weak acids mixed in with it since you're using grapefruit.  Ascorbic acid (vitamin C), etc. will all be problems for you.  The main problem is finding a way to separate the citric acid from the other weak acids.  You MIGHT be able to do it by selective precipitation (calcium citrate possibly?) and then using the solubility properties of the resulting compounds to separate them fairly well, but the resolution may not be so great.  Take a look on the 'net and see if you can figure out what acids are in grapefruit.  Then you'll have something to go on.

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