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Topic: Gravimetric Analysis Help  (Read 6415 times)

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Offline Fatpants15

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Gravimetric Analysis Help
« on: January 23, 2017, 03:01:41 PM »
Here's the problem from my homework. The amount of iron and manganese in an alloy can be determined by precipitating the metals with 8-hydroxyquinoline, C9H7NO (FW 145.16).  After weighing the mixed precipitate, the precipitate is dissolved and the amount of 8-hydroxyquinoline is determined by another method.  In a typical analysis, a 0.1273 g sample of an alloy containing iron, manganese, and other metals was dissolved in acid and treated with appropriate masking agents to prevent interference from other metals.  The iron and manganese were precipitated and isolated as Fe(C9H6NO)3 (FW 488.29) and Mn(C9H6NO)2 (FW 343.24), yielding a total mass of 0.8678 g.  The amount of 8-hydroxyquinoline in the mixed precipitate was determined to be 5.276 mmol.  Calculate the %(m/m) Fe and %(m/m) Mn in the alloy.

I'm not sure where to start because I was out of town due to family things. Any help is appreciated!

Offline Borek

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Re: Gravimetric Analysis Help
« Reply #1 on: January 23, 2017, 04:18:28 PM »
You should show your attempt to receive help, that's part of the forum rules.

Hint: you can write one equation using number of moles and a second equation using masses. That's enough to solve the problem.
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Offline Fatpants15

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Re: Gravimetric Analysis Help
« Reply #2 on: January 23, 2017, 11:25:30 PM »
Okay here's my work. I couldn't upload earlier and I didn't want to type it all out. Am I on the right track?

Offline Borek

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Re: Gravimetric Analysis Help
« Reply #3 on: January 24, 2017, 02:47:38 AM »
Your second equation is close, but the RHS is wrong.
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Offline Fatpants15

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Re: Gravimetric Analysis Help
« Reply #4 on: January 24, 2017, 08:44:27 PM »
What is the RHS? And how do I fix it?

Offline Borek

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Re: Gravimetric Analysis Help
« Reply #5 on: January 25, 2017, 03:09:15 AM »
RHS means Right Hand Side of the equation.

And how do I fix it?

You look at it wondering "What could have gone wrong?" ;)
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Offline Fatpants15

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Re: Gravimetric Analysis Help
« Reply #6 on: January 25, 2017, 03:09:46 PM »
Okay I think I got it now. Does this look good?

Offline Borek

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Re: Gravimetric Analysis Help
« Reply #7 on: January 25, 2017, 04:32:59 PM »
Isn't it the same picture?
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Offline Fatpants15

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Re: Gravimetric Analysis Help
« Reply #8 on: January 25, 2017, 10:15:50 PM »
Yeah it was supposed to post an updated one. Here's the updated one. I think I got it but I'm worried because my two percent masses add up to almost 100% but it said there were other metals present
« Last Edit: January 26, 2017, 03:28:22 AM by Borek »

Offline Borek

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Re: Gravimetric Analysis Help
« Reply #9 on: January 26, 2017, 03:42:35 AM »
Your equations are OK but your math is wrong.

Beware: this system of equation can be quite sensitive to rounding errors.

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