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Topic: Molarity problem  (Read 7130 times)

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Offline remeday86

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Molarity problem
« on: February 07, 2009, 05:11:51 AM »
Ascorbic acid (vitamin C) is a diprotic acid having the formula H2C6H6O6. A sample of a vitamin supplement was analyzed by titrating a 0.5054 g sample dissolved in water with 0.0713 M NaOH. A volume of 10.80 mL of the base was required to completely neutralize the ascorbic acid. What was the percentage by mass of ascorbic acid in the sample? ____% asorbic acid

--My work

0.0713 M NaOH = mole NaOH/ 0.0108 L = 7.7004 E -4 mol NaOH

7.7004 E -4 mol NaOH x ( 1 mol asorbic acid / 1 mol NaOH ) = 7.7004 E -4 mol ascorbic acid

7.7004 E -4 mol ascorbic acid x 176.1248 g/mol ascorbic acid = 0.1356 g ascorbic acid

(0.1356 g ascorbic acid / 0.5054 g sample) x 100% = 26.8 % ascorbic acid

but...its wrong. I really tried... :-\ Where did I go wrong? Please explain. Thanks in advance!

Offline Astrokel

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Re: Molarity problem
« Reply #1 on: February 07, 2009, 06:12:50 AM »
Quote
Ascorbic acid (vitamin C) is a diprotic acid having the formula H2C6H6O6.

Quote
7.7004 E -4 mol NaOH x ( 1 mol asorbic acid / 1 mol NaOH ) = 7.7004 E -4 mol ascorbic acid
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline aldoxime_amine

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Re: Molarity problem
« Reply #2 on: February 07, 2009, 10:04:51 AM »
This is only a suggestion, instead of working with moles and molarilty, it would be very helpful if you understood the concept of equivalents and normality.

Offline remeday86

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Re: Molarity problem
« Reply #3 on: February 07, 2009, 11:37:11 PM »
Quote
Ascorbic acid (vitamin C) is a diprotic acid having the formula H2C6H6O6.

Quote
7.7004 E -4 mol NaOH x ( 1 mol asorbic acid / 1 mol NaOH ) = 7.7004 E -4 mol ascorbic acid


Thanks!

7.7004 E -4 mol NaOH x ( 2 mol asorbic acid / 1 mol NaOH ) = 1.5401 E -3 mol ascorbic acid

1.5401 E -3 mol ascorbic acid x 176.1248 g/mol ascorbic acid = 0.2712 g ascorbic acid

(0.2712 g ascorbic acid / 0.5054 g sample) x 100% = 53.7% ascorbic acid

....but i still got it wrong. :-[

Offline Astrokel

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Re: Molarity problem
« Reply #4 on: February 08, 2009, 03:35:07 AM »
Quote
2 mol asorbic acid / 1 mol NaOH
Think again, every 1 mole of acid has 2 moles of H+. So if you use 2 moles of acid, you have 4 moles of H+ but you use only 1 mole of base which has only 1 mole of OH-. It can't neutralise.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

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