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Topic: Gravimetric Analysis  (Read 9892 times)

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Offline kizuna

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Gravimetric Analysis
« on: March 15, 2008, 09:04:08 AM »
Hi all. I was wondering if someone could help me with this question?

"Reconcile the molar ratio obtained in question 6 with the molecular formula of the material in the crucible before it was heated."

*molar ratio was worked out to be 1.0116 (answer to question 6. molar ratio between H2O and BaCl2)
*molecular formula is BaCl2.2H2O

I don't understand what this question is asking, so any help is greatly appreciated.

What the experiment involved was heating hydrated barium chloride (BaCl2.2H2O) to get anhydrated barium chloride (BaCl2)


Just an extra question: when i work out the molar mass of water lost, do I multiply it by two, like the formula? I'm not sure if it should be 18 or 36 g mol-1
« Last Edit: March 15, 2008, 09:48:12 AM by kizuna »

Offline Borek

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Re: Gravimetric Analysis
« Reply #1 on: March 15, 2008, 09:39:35 AM »
Hard to answer your question, not knowing what question 6 asked for.

36g/mol is lost.
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Offline Arkcon

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Re: Gravimetric Analysis
« Reply #2 on: March 15, 2008, 09:41:50 AM »

Just an extra question: when i work out the molar mass of water lost, do I multiply it by two, like the formula? I'm not sure if it should be 18 or 36 g mol-1

Now this edited question we can answer, yes, you do multiply the coefficient.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline kizuna

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Re: Gravimetric Analysis
« Reply #3 on: March 15, 2008, 09:51:41 AM »
Terribly sorry, I edited the question again.
Question 6 just wanted to know the molar ration between n(H2O) and n(BaCl2) which I calculated to be 1.0116

Offline Borek

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Re: Gravimetric Analysis
« Reply #4 on: March 15, 2008, 10:15:21 AM »
Either I don't understand something, or you did something wrong. If the compound was BaCl2.2H2O and you heated it to get anhydrous BaCl2, molar ratio should be 0.5 (or 2), but not around 1...
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Offline kizuna

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Re: Gravimetric Analysis
« Reply #5 on: March 15, 2008, 10:41:36 AM »
My results from the experiment were:
mass of BaCl2.2H2O = 0.9804 g
mass of BaCl2 = 0.8345 g
mass of H2O lost = 0.9804 - 0.8345 = 0.1459 g

n1(H2O) = 0.1459/36 = 4.0528 x 10-3 mol
n2(BaCl2) = 0.8345/208.3 = 4.0062 x 10-3 mol

molar ratio = n1/n2
4.0528 x 10-3/4.0062 x 10-3 = 1.0116

Have I done something wrong?

Offline Borek

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Re: Gravimetric Analysis
« Reply #6 on: March 15, 2008, 11:49:07 AM »
n1(H2O) = 0.1459/36 = 4.0528 x 10-3 mol

What is molar mass of water?
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Offline Arkcon

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Re: Gravimetric Analysis
« Reply #7 on: March 15, 2008, 12:02:13 PM »
n1(H2O) = 0.1459/36 = 4.0528 x 10-3 mol

What is molar mass of water?

Yes, disregard my previous advice and try to see if you get the correct molar ratio.  Srry.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline kizuna

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Re: Gravimetric Analysis
« Reply #8 on: March 15, 2008, 01:07:34 PM »
ok, if I use 18 as the molar mass for water, I should get the right molar ratio.

n1(H2O) = 0.1459/18 = 8.1056 x 10-3 mol

molar ratio = n1/n2
8.1056 x 10-3/4.0062 x 10-3 = 2.0233

Is this correct?

Offline Borek

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Re: Gravimetric Analysis
« Reply #9 on: March 15, 2008, 01:33:14 PM »
Yes. It tells you what formula of hydrate is.
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Offline kizuna

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Re: Gravimetric Analysis
« Reply #10 on: March 15, 2008, 01:43:33 PM »
H2?

I'm afraid this is where I get a bit lost. I don't know what the question wants me to do.

Offline Borek

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Re: Gravimetric Analysis
« Reply #11 on: March 15, 2008, 02:32:31 PM »
I believe you are asked to compare your experimental result (which is close to 2) with the theoretical value (which is just 2).

And why H2? You meant H2O?
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Offline kizuna

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Re: Gravimetric Analysis
« Reply #12 on: March 16, 2008, 04:34:31 AM »
Yes, I did mean H2O, sorry.

Well thank you very much for your help ;D

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