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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: kizuna on March 15, 2008, 09:04:08 AM

Title: Gravimetric Analysis
Post by: kizuna 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
Title: Re: Gravimetric Analysis
Post by: Borek on March 15, 2008, 09:39:35 AM: Hard to answer your question, not knowing what question 6 asked for.

36g/mol is lost.
Title: Re: Gravimetric Analysis
Post by: Arkcon on March 15, 2008, 09:41:50 AM: Quote from: kizuna on March 15, 2008, 09:04:08 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.
Title: Re: Gravimetric Analysis
Post by: kizuna 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
Title: Re: Gravimetric Analysis
Post by: Borek 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...
Title: Re: Gravimetric Analysis
Post by: kizuna 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?
Title: Re: Gravimetric Analysis
Post by: Borek on March 15, 2008, 11:49:07 AM: Quote from: kizuna on March 15, 2008, 10:41:36 AM
n₁(H₂O) = 0.1459/36 = 4.0528 x 10^-³ mol

What is molar mass of water?
Title: Re: Gravimetric Analysis
Post by: Arkcon on March 15, 2008, 12:02:13 PM: Quote from: Borek on March 15, 2008, 11:49:07 AM
Quote from: kizuna on March 15, 2008, 10:41:36 AM
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.
Title: Re: Gravimetric Analysis
Post by: kizuna 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?
Title: Re: Gravimetric Analysis
Post by: Borek on March 15, 2008, 01:33:14 PM: Yes. It tells you what formula of hydrate is.
Title: Re: Gravimetric Analysis
Post by: kizuna 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.
Title: Re: Gravimetric Analysis
Post by: Borek 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?
Title: Re: Gravimetric Analysis
Post by: kizuna on March 16, 2008, 04:34:31 AM: Yes, I did mean H₂O, sorry.

Well thank you very much for your help ;D