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Chemistry homework *DELETED*
    #8663878 -

Post deleted by SubGen1us

Reason for deletion: .


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Re: Chemistry homework [Re: SubGen1us]
    #8663894 -

Assuming you want percent by mass:

1.11+1.09+.61=2.81g total

NH4Cl: 1.11/2.81 = 40%
SiO2: 1.09/2.81 = 39%
NaCl: .61/2.81 = 22%

You'd probably want more significant figures than that though. And if you need molar percent, convert them to moles and do the same calculation.


--------------------
I know... that just the smallest
                                                part of the world belongs to me
You know... I'm not a blind man
                                                    but truth is the hardest thing to see

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Re: Chemistry homework [Re: SubGen1us]
    #8663895 -

% of mass or % of mols?


edit - ya beat me!  My first thought was he wanted a mol percentage.

Edited by Qubit (07/21/08 04:41 PM)

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Re: Chemistry homework *DELETED* [Re: DieCommie]
    #8663909 -

Post deleted by SubGen1us

Reason for deletion: .


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Re: Chemistry homework [Re: SubGen1us]
    #8664005 -

I would convert to moles then do the math, as this will give you a more accurate estimate.

OK did the math:

1.1g NH4Cl = 20 millimols
1.09g SiO2 = 18 millimols
0.61g NaCl = 10 millimols
            ______________
            48 millimols total

just divide each by 48.  Numbers I get:

NH4Cl = 41.6%
SiO2 = 37.5%
NaCl = 20.8%

Edited by J6000 (07/21/08 05:22 PM)

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Re: Chemistry homework [Re: J6000]
    #8664022 -

It wont give him a more accurate estimate of the mass percentage, which is what he needs!

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Re: Chemistry homework [Re: DieCommie]
    #8664073 -

DieCommie said:
It wont give him a more accurate estimate of the mass percentage, which is what he needs!


Uhhh...moles = molar mass.  It is more accurate.  You cant just crunch the numbers of the grams because grams are a weight, and carry no mass information.  You need grams and molecular weight to give accurate percentages.

Ever taken an organic chem lab?  Using grams instead of moles can spell disaster! 1g HCl will not neutralize 1g NaOH.  Just because they are equal by weight does not mean it's a 50/50 split!

Edited by J6000 (07/21/08 05:42 PM)

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Re: Chemistry homework [Re: J6000]
    #8664088 -

Gram is a measure of mass not weight. 

He wants percentage by mass, not by moles.  He is not trying to balance an equation or neutralize anything.  He simply wants percentage by mass (not moles).

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Re: Chemistry homework [Re: DieCommie]
    #8664134 -

Mol% =! Mass%


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Re: Chemistry homework [Re: Minstrel]
    #8664146 -

That's why I hate it when chemical recipes use percentage without specifying if it's w/w, m/m, v/v or w/v. They are all different.


--------------------
I know... that just the smallest
                                                part of the world belongs to me
You know... I'm not a blind man
                                                    but truth is the hardest thing to see

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Re: Chemistry homework [Re: zouden]
    #8664177 -

Ah, I see what you are all saying.  Its been 3 years since ochem, and I am a bio major. (although I also already have a major in chemistry :undecided:)

Although I will throw in there that every time I came across a question like this I always used molar mass instead of grams and always got it right.

Doesn't the question seem too simple if the correct answer is truly grams/total grams?  Its like asking:

You've got 3 dogs and 1 cat.  What percentage are cats?

Unless this is a highschool class...?

Edited by J6000 (07/21/08 06:06 PM)

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Re: Chemistry homework [Re: zouden]
    #8664187 -

zouden said:
That's why I hate it when chemical recipes use percentage without specifying if it's w/w, m/m, v/v or w/v. They are all different.


I agree with you 100% on that one, although I have never seen a chemistry recipe using percentages!  Always grams (preferable) or molarity of final concentration. 

Who makes recipes with percentages?!?!

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Re: Chemistry homework [Re: J6000]
    #8664208 -

It is completely normal to ask for % of mass or % of moles in freshman college chem classes.

EDIT
Quote:
Doesn't the question seem too simple if the correct answer is truly grams/total grams?  Its like asking:

You've got 3 dogs and 1 cat.  What percentage are cats?



Well, if you were doing something like shipping the animals, their mass percentage would be more important than there mole percentage.  Each has a use in different contexts.

Edited by Qubit (07/21/08 06:08 PM)

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Re: Chemistry homework [Re: J6000]
    #8665576 -

J6000 said:
zouden said:
That's why I hate it when chemical recipes use percentage without specifying if it's w/w, m/m, v/v or w/v. They are all different.


I agree with you 100% on that one, although I have never seen a chemistry recipe using percentages!  Always grams (preferable) or molarity of final concentration. 

Who makes recipes with percentages?!?!


Usually informal type things, like someone will write down their recipe for a certain media on a post-it note and it'll say "3% glucose".


--------------------
I know... that just the smallest
                                                part of the world belongs to me
You know... I'm not a blind man
                                                    but truth is the hardest thing to see

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