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Offline ryan1

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concentration question
« on: November 03, 2009, 03:56:19 PM »
Hi. I'm having trouble with this:

In order to standardise a solution of sodium hyrdoxide, a chemists first prepared a solution of ethanedioic acid-2-water, H2C2O4 . 2H2O by dissolving 14.6g of crystals in water and making the solution up to 250 cm3 in a graduated flask. He then pipetted 25cm3 of this solution into a conical flask, added phenolphthalein solution as indicator, and titrated it against the sodium hydroxide solution! 24.1cm3 of the latter were required. Calculate, first, the molar concentratoin of the ethanedioic acid solution, and then that of the sodium hydroxide.


My attempt:


moles H2C2O4 . 2H2: 14.6/ 126 = 0.116 moles
concentration of H2C2O4.2H2O: 0.116 / (25/1000) = 4.64 moles
H2C2O4.2H2O + NaOH --> NaHC2O4 + 3H2O ( 1:1 reaction)

4.64/ (24.1 / 1000) = 192.5 mold dm3 (THIS LOOKS LIKE A VERY HIGH CONCENTRATION)

can someone please check it and tell me where I went wrong and guidance on how to get the right answer please

Offline DrCMS

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Re: concentration question
« Reply #1 on: November 03, 2009, 04:20:08 PM »
How much oxalic acid was dissolved in how much water?
How much of that solution was taken for the titration?
So how much oxalic acid was titrated against the NaOH?

Offline ryan1

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Re: concentration question
« Reply #2 on: November 03, 2009, 04:30:56 PM »
How much oxalic acid was dissolved in how much water?
How much of that solution was taken for the titration?
So how much oxalic acid was titrated against the NaOH?

14.6g was dissolved in water.

one tenth of this was used for titration.... so  i guess you need to used 0.0116 moles.

if this is the case:

0.0116/(25/1000) = 0.464 mol dm3 of oxalic acid.


therefore, the conc. of NaOH : 0.0116/ (24.1/1000) = 0.481 mol dm3


this looks better, is it right?

Offline csrscience.com

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Re: concentration question
« Reply #3 on: November 03, 2009, 07:19:33 PM »
oxalic acid is a diprotic acid and as such it will take two sodium hydroxides to neutralize one oxalic acid.

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Offline ryan1

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Re: concentration question
« Reply #4 on: November 04, 2009, 02:07:36 PM »
what does thatmean? 2x the amount of moles: 0.0232 NaOH

Offline ryan1

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Re: concentration question
« Reply #5 on: November 04, 2009, 02:21:35 PM »
I DONT UNDERSTAND

all i know that is correct now is there are 0.116 moles in ethanedioic acid-2-water.


now from here im confused. i did:0.116/0.25 = 0.464 mol dm3 (in 250cm3)
conc. in 25cm3 = 0.0464cm3


please dont reply wiht confusing things, i prob could have completed this a lot more quickly if the replies werent so cryptic

Offline Borek

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Re: concentration question
« Reply #6 on: November 04, 2009, 03:00:55 PM »
I DONT UNDERSTAND

all i know that is correct now is there are 0.116 moles in ethanedioic acid-2-water.


now from here im confused. i did:0.116/0.25 = 0.464 mol dm3 (in 250cm3)

Almost OK (molarity is not mol*dm3, but mol/dm3).

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conc. in 25cm3 = 0.0464cm3

This is wrong, for two reasons - concentration doesn't depend on the volume, concentration is not measured in cm3. You are probably referring to the number of moles of oxalic acid in 25 mL of the solution.

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please dont reply wiht confusing things, i prob could have completed this a lot more quickly if the replies werent so cryptic

They were not cryptic and they both pointed exactly to your errors. Unfortunately, you seem to be completely lost and confused with things like volumes, concentrations, number of moles and stoichiometry - you should use them without problems if you want to solve titration questions.
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Offline ryan1

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Re: concentration question
« Reply #7 on: November 04, 2009, 03:24:37 PM »
nobody is telling me WHAT to do. I have plenty of replys saying what i have done wrong.

i guess the moles would reduce by a tenth so in 25cm3 there would be 0.0116 moles. from here u can calculate the concentration which would be the same.
 

also if this amount of moles is correct, when it comes to finding the number of moles of NaOH would I use 0.0116 moles for the 25cm3 solution or 0.116 for the 250cm3 solution?

i am not sure if this is right so far, but thanks for the previous reply. it will take me a long time to deduce the answer from all of my mistakes though

Offline ryan1

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Re: concentration question
« Reply #8 on: November 04, 2009, 03:38:38 PM »
if it is right to use the number of moles of oxalic acid to be 0.0116 moles in 25cm3 i did this:

moles NaOH = 0.0232 moles (1:2)


conc. is 0.0232/(24.1/1000) = 0.962 mol dm-3


is this right????????

Offline DrCMS

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Re: concentration question
« Reply #9 on: November 04, 2009, 03:56:02 PM »
Yes but do you understand why it is right and where you went wrong earlier.

Offline Borek

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Re: concentration question
« Reply #10 on: November 04, 2009, 04:01:13 PM »
nobody is telling me WHAT to do.

Surprise! It is YOUR homework and you have to do it. We can help, but telling you "now multiply by two and add 7" won't teach you anything.

Quote
also if this amount of moles is correct, when it comes to finding the number of moles of NaOH would I use 0.0116 moles for the 25cm3 solution or 0.116 for the 250cm3 solution?

0.0116 - that's number of moles in the aliquot transferred to the conical flask and titrated. Rest sits safely in volumetric flask and waits for other uses.
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