Solving for Grams of Citric Acid in Juice Solution

[jumbogala]

[SOLVED] Titration Lab

Homework Statement

How many grams of pure citric acid in the juice solution when 4.00 g of the juice crystals are dissolved in 100.0 mL solution?

The juice solution is titrated using a given solution of sodium hydroxide. The NaOH is titrated into 10 mL of the juice solution at a time. Phenolphthalein is used as an indicator.

Citric acid is 192.14 g/mol.

The NaOH is 0.10 mol/L.

Let's assume I had to use 50 mL of NaOH for the phenolphthalein to turn pink.

Homework Equations

Not sure?

The Attempt at a Solution

Unfortunately it's been a year and half since I did my last titration and I can't remember how to do this at all. I remember that I have to do a balanced chemical equation, but seeing as I don't know the chemical formula for citric acid I can't do that.

I know that I had to add 0.005 mol of NaOH to the solution. (0.05 L * 0.10 mol/L NaOH).

Also if 4 g crystals/100 mL of juice solution, then in 10 mL of the juice solution there would be 0.4 g of crystals. But how does any of this let me find the mol/L of acid??

 

[cemar.]

ahhh the cursed tang titration.
Start with what you know for real dude. A balanced chemical reaction crucial (you can get chemical formulas off google... or in your textbook =O)
so when you get that down think about what is totally and completely one hundred percent unquestionable. You say you know exactly how much NaOH you needed?? SWEET.
SO!
multiply volume of NaOH by concentration (only of what was used in titration, sounds easy but people get confused sometimes). This will give you moles of NaOH!
Then you can use your molar ratio of NaOH to citric acid to find moles of citric acid in your erlenmeyer flask.

**Chem is sweet because units always cancel to give you the units you're looking for so you always know when you are right! COOL RIGHT?!?**

Now you have moles of citric acid so go nuts with your molar mass and get the grams. This will tell you how many grams were present in the 10 mL of the juice you used.

 

[jumbogala]

Thanks so much for replying quickly!

I found a second sheet that goes with this lab that says that citric acid is H3Ct.

But I don't understand how to get the balanced chemical equation, since I don't know what citric acid and NaOH react to make. And without the balanced chemical equation, I can't get the molar ratio, right?

I do know the mol wt. of citric acid, does that help?

 

[cemar.]

Yah i definitely did this lab once upon a time.
So as far as i know the UNBALANCED equation should look something like this:
H3Ct + NaOH ---> NaCt + H2O
And you are correct that without balancing it you will not get the molar ratio.
(predicting chemical equations should get pretty easy. Think DOUBLE REPLACEMENT it becomes second nature and you just get and eerie intuition how it goes. an acid and base together almost always produce water)
if by molar weith you mean 192.14 g/mol then that's what going to get you from moles of H3Ct to grams.

 

[cemar.]

and you wouldn't happen to know any thing about currents and all that physics resistance voltage jazz would you?? phyisics is definietly the bane of my existence..

 

[jumbogala]

Ooh, it makes sense now. Thank you again!

I can try to answer physics questions if they're high school level. If they are, why don't you message me any questions you have?

Also you can try http://www.glenbrook.k12.il.us/gbssci/Phys/Class/BBoard.html" under static and current electricity, I've always found that site very helpful.

 

[MichaelXY]

I am sure I am going to feel silly for asking this, but what the heck is Ct? I thought Citric acid was C6H8O7. Is Ct some kind of abbreviated way of writing the formula?

 

[chemisttree]

In this case it means citrate. H3Ct is citric acid and NaCt is sodium citrate. It isn't standard notation but it works for Jumbogola in this case so who can criticize?

 

[MichaelXY]

Thanks for clearing that up for me. I did not mean to sound critical, rather I never seen that before and wished to be enlightened. Thanks :)

 

[cemar.]

Thanks so much! i got er under control.
Howd the lab write up go??

 

[jumbogala]

I haven't done it yet, it's not due until Wednesday. I wanted to ask my question early though to make sure I could get an answer in time.

Thanks again!

 

[jumbogala]

Okay so now we're doing extension questions on the lab, and we have to find 3 sources of error. I'm having trouble wording them and wondering if these sound okay:

- Since it is difficult to tell when the endpoint of the titration has been reached, the point where the titration was stopped may have varied from trial to trial.
-Since it is difficult to get exactly 10.0 mL of juice solution for each trial, the amount of juice solution used in each trial may have varied very slightly between trials.
- The buret tube adds drops of NaOH to the juice solution at a time. Near the end point it's possible that less than a drop would be required for the juice solution to change color. (especially this one, how can I word this to make it sound clearer?)

We also need to state possible improvements to the lab. Do you think I need to state improvements with respect to the errors? Because some of these errors are things that can't be fixed...

 

[opuktun]

- Since it is difficult to tell when the endpoint of the titration has been reached, the point where the titration was stopped may have varied from trial to trial.
Not true. Strong Base - Weak Acid titration does not cause this.

-Since it is difficult to get exactly 10.0 mL of juice solution for each trial, the amount of juice solution used in each trial may have varied very slightly between trials.
Not really true. You can blame this on "experimenter's technique" (i.e. you) ;p It's ok to do so. For instance, say something like:

Errors could be attributed to the experimenter's technique. Transfer of solutions during pipetting or some solution not properly pipetted out may result in some errors.

Experimenters are not perfect...

- The buret tube adds drops of NaOH to the juice solution at a time. Near the end point it's possible that less than a drop would be required for the juice solution to change color.
I will still blame it on experimenter's technique. Say something like:
The inability to dispense appropriate amount of titrant from the burette may result in errors as well.

We also need to state possible improvements to the lab. Do you think I need to state improvements with respect to the errors? Because some of these errors are things that can't be fixed...
Like?