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### Topic: Percent Composition Lab Procedure  (Read 14184 times)

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#### kaptemplar

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##### Percent Composition Lab Procedure
« on: March 21, 2006, 09:13:29 PM »
can anyone outline a laboratory procedure that can be used to determine the mass percent composition of an alloy of copper and silver. the alloy dissolves completely in concentrated nitric acid.

I am just interested in the procedure...I can probably figure out the type of calculations and what not myself.

Thank you

#### tonyliruhan

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##### Re:Percent Composition Lab Procedure
« Reply #1 on: March 21, 2006, 09:54:40 PM »
add water to make the solution dilute
and put some NaCl to produce AgCl
meassure the mass and get the mass of Ag
and problem solved

#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #2 on: March 21, 2006, 10:18:15 PM »
so the precipitate AgCl, how would you separate the Ag from the Cl? a Single replacement Reaction?

#### AWK

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##### Re:Percent Composition Lab Procedure
« Reply #3 on: March 22, 2006, 01:53:49 AM »
so the precipitate AgCl, how would you separate the Ag from the Cl? a Single replacement Reaction?
On the basis of calculations
AWK

#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #4 on: March 22, 2006, 02:46:02 AM »
how would you that? I think I have an idea involving some stoichiometry, does that sound correct to you?

#### Borek

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##### Re:Percent Composition Lab Procedure
« Reply #5 on: March 22, 2006, 04:06:04 AM »
how would you that? I think I have an idea involving some stoichiometry, does that sound correct to you?

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#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #6 on: March 22, 2006, 02:49:59 PM »
ummm you do

mass of AgCl from the exp * (1 mol AgCl/MW of AgCl) * (1 mol Ag / 1 mol AgCl) * ( MW weight of AgCl / 1 mol Ag)

is that right?

#### Borek

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##### Re:Percent Composition Lab Procedure
« Reply #7 on: March 22, 2006, 04:55:58 PM »
No. Everything cancels out and you are left with mass of AgCl - the same you have started with.

Think about it in terms of ratios.

MW of AgCl is 143.3g, MW of Ag is 107.9g.

It means that every 143.3g AgCl contains 107.9g of Ag.

and every m g AgCl contains x g Ag

143.3/m = 107.9/x

Solve for x.
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#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #8 on: March 22, 2006, 08:59:42 PM »
ah yeah that's what I meant just a type in the end, it should have the MW of Ag. k I follow your idea, the separation, the math, I have one last question though. The material is an alloy with 2 metals, so it's chemical equation is something like Ag2Cu?

so I would have to write

Ag2Cu + NaCl ----> AgCl(s) + Na2Cu

and then

Na2Cu + Li3PO4 ---> Cu3(PO4)2 (S) + NaLi

is that it for the chemical equations?

#### AWK

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##### Re:Percent Composition Lab Procedure
« Reply #9 on: March 23, 2006, 01:28:05 AM »
ah yeah that's what I meant just a type in the end, it should have the MW of Ag. k I follow your idea, the separation, the math, I have one last question though. The material is an alloy with 2 metals, so it's chemical equation is something like Ag2Cu?

so I would have to write

Ag2Cu + NaCl ----> AgCl(s) + Na2Cu

and then

Na2Cu + Li3PO4 ---> Cu3(PO4)2 (S) + NaLi

is that it for the chemical equations?

Alloy Ag2Cu does not react with NaCl

Alloy Ag2Cu does not react with NaCl

Start from dissolving of both metals (weighted sample of an alloy) in concentrated HNO3
Then precipitate AgCl and weight it after drying.
These data will be sufficient to calculate mass percent composition of this alloy
AWK

#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #10 on: March 23, 2006, 03:36:22 AM »
that makes sense, the problem hints that the alloy dissolves in HNO3

but then what would the chemical reaction involving the AgCl, the alloy and the HNO3 be....

#### Borek

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##### Re:Percent Composition Lab Procedure
« Reply #11 on: March 23, 2006, 03:44:32 AM »
that makes sense, the problem hints that the alloy dissolves in HNO3

but then what would the chemical reaction involving the AgCl, the alloy and the HNO3 be....

These are separate reactions - first is alloy dissolution (which on its own is two separate reactions, one of copper dissolution and one of silver dissolution), second is the silver chloride precipitation.
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#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #12 on: March 23, 2006, 03:57:01 AM »
and what would the chemical reaction for the 2 dissolutions be?

something like

Ag2cu(s) + Hno3(Aq) ----> Ag(aq) + Cu(aq) + HNO3 (aq)

#### AWK

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##### Re:Percent Composition Lab Procedure
« Reply #13 on: March 23, 2006, 04:11:33 AM »
Ag(s) + 2HNO3 = AgNO3 + NO2(g) + H2O
Cu(s) + 4HNO3 = Cu(NO3)2 + 2NO2(g) + 2H2O
AWK

#### kaptemplar

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##### Re:Percent Composition Lab Procedure
« Reply #14 on: March 23, 2006, 04:15:21 AM »
oh I see, I have one last question then, I have never done this in the lab and it's an extra credit problem so I dont really know much about alloys so my question is can you actually tell the difference between Ag and Cu in the alloy? I mean isnt the alloy just a whole bunch of solid in one color? cuz if it is how would you make those 2 reactions happen