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Topic: Finding Weight Percent in a Compound (check)  (Read 9422 times)

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Offline Hemidol

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Finding Weight Percent in a Compound (check)
« on: October 08, 2009, 12:19:22 AM »
A mixture of Zn3(PO4)2 and its hydrate Zn3(PO4)2•4H2O has a mass of 1.9497 g. After heating to drive
off all the water, the mass is only 1.8311 g. What is the weight percentage of Zn3(PO4)2•4H2O in the original
mixture? Report your answer to 4 significant figures.
Molar masses: Zn3(PO4)2 386.11 g/mol; Zn3(PO4)2•4H2O 458.17 g/mol.


Here is my work...

1.9497g - 1.8311g = 0.1186g


(386.11/72.06) x 0.1186 g = 0.635479406


Then:

0.1186g + 0.635479406g = 0.7541g

0.7541g * 100% = 75.41 %

Is this the correct technique?
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Offline Borek

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Re: Finding Weight Percent in a Compound (check)
« Reply #1 on: October 08, 2009, 03:41:24 AM »
You have correctly found mass of hydrate, but 75% is not the correct result.
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Offline Hemidol

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Re: Finding Weight Percent in a Compound (check)
« Reply #2 on: October 08, 2009, 10:27:25 AM »
Quote from: Borek on October 08, 2009, 03:41:24 AM
You have correctly found mass of hydrate, but 75% is not the correct result.
Where did I go wrong? What will the correct perrcent be?


Thanks for finding this correction.
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Offline AWK

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Re: Finding Weight Percent in a Compound (check)
« Reply #3 on: October 08, 2009, 10:37:35 AM »
Quote
0.7541g * 100% = 75.41 %
this step is wrong
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Offline Hemidol

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Re: Finding Weight Percent in a Compound (check)
« Reply #4 on: October 08, 2009, 10:51:19 AM »
Quote from: AWK on October 08, 2009, 10:37:35 AM
Quote
0.7541g * 100% = 75.41 %
this step is wrong
Ah, so:

0.635479406 / 0.7541g * 100 = 84.27

Better? :)
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Offline DrCMS

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Re: Finding Weight Percent in a Compound (check)
« Reply #5 on: October 08, 2009, 11:45:44 AM »
Quote from: Hemidol on October 08, 2009, 10:51:19 AM
Quote from: AWK on October 08, 2009, 10:37:35 AM
Quote
0.7541g * 100% = 75.41 %
this step is wrong
Ah, so:

0.635479406 / 0.7541g * 100 = 84.27

Better? :)

No worse.

You have correctly calculated the weight of hydrate in the sample as 0.7541g.

What was the weight of the sample?

So what is the % of hydrate in the sample?
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Offline Hemidol

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Re: Finding Weight Percent in a Compound (check)
« Reply #6 on: October 08, 2009, 12:14:59 PM »
Weight of sample = 1.9497 g

% of hydrate = 0.7541g * 1.9497 ?

Sorry this question really is boggling me.  ???
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Offline Borek

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Re: Finding Weight Percent in a Compound (check)
« Reply #7 on: October 08, 2009, 12:40:40 PM »
Edit: oops.  :-[
« Last Edit: October 08, 2009, 01:59:10 PM by Borek »
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Offline DrCMS

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Re: Finding Weight Percent in a Compound (check)
« Reply #8 on: October 08, 2009, 01:39:45 PM »
Quote from: Hemidol on October 08, 2009, 12:14:59 PM
Weight of sample = 1.9497 g

% of hydrate = 0.7541g * 1.9497 ?

No the % of hydrate = 0.7541 / 1.9497

Your maths teacher should be shot.
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Offline Hemidol

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Re: Finding Weight Percent in a Compound (check)
« Reply #9 on: October 08, 2009, 02:25:16 PM »
Quote from: DrCMS on October 08, 2009, 01:39:45 PM
Quote from: Hemidol on October 08, 2009, 12:14:59 PM
Weight of sample = 1.9497 g

% of hydrate = 0.7541g * 1.9497 ?

No the % of hydrate = 0.7541 / 1.9497

Your maths teacher should be shot.
This gives a percent of 0.3867... that doesn't seem correct. That tiny?

I'm assuming I'll then times this value by 100% :P?
« Last Edit: October 08, 2009, 02:41:10 PM by Hemidol »
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Offline DrCMS

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Re: Finding Weight Percent in a Compound (check)
« Reply #10 on: October 08, 2009, 04:14:36 PM »
Quote from: Hemidol on October 08, 2009, 02:25:16 PM
Quote from: DrCMS on October 08, 2009, 01:39:45 PM
Quote from: Hemidol on October 08, 2009, 12:14:59 PM
Weight of sample = 1.9497 g

% of hydrate = 0.7541g * 1.9497 ?

No the % of hydrate = 0.7541 / 1.9497

Your maths teacher should be shot.
This gives a percent of 0.3867... that doesn't seem correct. That tiny?

I'm assuming I'll then times this value by 100% :P?

Yes the correct answer is 38.67% and I was wrong actually your maths teacher should commit suicide in shame.
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