Homework Help: Chemistry

Posted by Adam-- Dr Bob, Help! on Wednesday, September 26, 2012 at 5:34pm.

I Posted this question the other day:

A 1.00 g sample of enriched water, a mixture of H2O and D2O, reacted completely with Cl2 to give a mixture of HCl and DCl. The HCl and DCl were then dissolved in pure H2O to make a 1.00 L solution. A 25.00 mL sample of the 1.00 L solution was reacted with excess AgNO3 and 0.3800 g of an AgCl precipitate formed.
What was the mass % of D2O in the original sample of enriched water?

your reply was :

I have deleted the original response and replaced it with the following. Check my thinking.

Cl2 + H2O ==> HOCl + HCl
Cl2 + D2O ==> DOCl + DCl

let x = mass H2O
and y = mass D2O
------------------
x + y = 1.00
[x*(molar mass AgCl/molar mass H2O)] + [y*(molar mass AgCl/molar mass D2O)] = 0.3800 x 1000/25

Two equation in two unknowns. Solve for x and y and convert to percent. Check my thinking.

---

I understand your reasoning and I feel that this should work, but when I try to solve it I am confused. My two equations would be :

x + y = 1.00

15.2=7.96x + 7.16y

when I solve

x= 10.05 and y = -9.05

I'm not sure if this is wrong or if I just don't understand the concept. When I would calculate the % D2O it doesn't really make sense...

Chemistry - DrBob222, Wednesday, September 26, 2012 at 6:56pm
Thanks for posting the original question and my response. I don't get your numbers exactly but I agree something is wrong because, I, too, obtained a negative number for D2O and that can't be. I have looked at my solution and I don't see anything that stares out at me that I can correct.
Does the problem give any additional information? Do you have an answer? Is there an equation for the reaction with Cl2? Have you checked that the problem is posted correctly with all of the correct numbers?

There is something very basic that's wrong for the following:
If I take the 0.3800 g AgCl, convert to moles and back to mols H2O and convert that to grams I get 1.9g if it's 100% H2O or 2.17 g if it's 100% D2O. And neither of those can be right since we had only 1.00 g total initially.
Chemistry - Adam-- Dr Bob, Help!, Wednesday, September 26, 2012 at 7:24pm
Deuterium Oxide (D2O) is a commonly solvent for NMR spectroscopy. However, it is often
contaminated with H2O. To test for residual H2O in D2O , a student took 1.00g of D2O + H2O
sample and reacted with Cl2 to give a mixture of HCl and DCl. The HCl and DCl mixture was
then dissolved to make a 1.00L solution. A 25mL sample of the 1.00L solution was reacted with
excess AgNO3 and 0.3800g of AgCl formed. What was the mass % of D2O in the original
sample of deuterium enriched water?

that is the exact question, I don't think I missed anything, but maybe you will see something different
Chemistry - DrBob222, Wednesday, September 26, 2012 at 10:35pm
I have turned this thing around in my head and I don't see anything wrong but it is obvious that SOMETHING is wrong. Either I've done something that isn't cricket or one of the numbers in the problem is not right. I will ask my friend, Bob Pursley, to take a look at it. If you will post each day I will have a way to get back to you.
Chemistry - bobpursley, Thursday, September 27, 2012 at 4:04am
I dont see anything wrong with the concept in the solution. Obviously, with a negative solution, it means probably that one of the data must be wrong
Chemistry - Adam-- Dr Bob, Help!, Thursday, September 27, 2012 at 9:44am
Thank you both so much for your help , it is really appreciated.

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