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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Hemidol on October 08, 2009, 12:19:22 AM

Title: Finding Weight Percent in a Compound (check)
Post by: Hemidol 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?

Title: Re: Finding Weight Percent in a Compound (check)
Post by: Borek on October 08, 2009, 03:41:24 AM

You have correctly found mass of hydrate, but 75% is not the correct result.

Title: Re: Finding Weight Percent in a Compound (check)
Post by: Hemidol 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.

Title: Re: Finding Weight Percent in a Compound (check)
Post by: AWK on October 08, 2009, 10:37:35 AM

Quote

0.7541g * 100% = 75.41 %

this step is wrong

Title: Re: Finding Weight Percent in a Compound (check)
Post by: 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? :)

Title: Re: Finding Weight Percent in a Compound (check)
Post by: DrCMS 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?

Title: Re: Finding Weight Percent in a Compound (check)
Post by: Hemidol 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.  ???

Title: Re: Finding Weight Percent in a Compound (check)
Post by: Borek on October 08, 2009, 12:40:40 PM

Edit: oops.  :-[

Title: Re: Finding Weight Percent in a Compound (check)
Post by: 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.

Title: Re: Finding Weight Percent in a Compound (check)
Post by: 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?

Title: Re: Finding Weight Percent in a Compound (check)
Post by: DrCMS 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.