Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: kaptemplar 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

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

so the precipitate AgCl, how would you separate the Ag from the Cl? a Single replacement Reaction?

so the precipitate AgCl, how would you separate the Ag from the Cl? a Single replacement Reaction?
On the basis of calculations

how would you that? I think I have an idea involving some stoichiometry, does that sound correct to you?

how would you that? I think I have an idea involving some stoichiometry, does that sound correct to you?
Probably. Show your approach and we will help you.

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?

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.

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?

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

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....

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.

and what would the chemical reaction for the 2 dissolutions be?
something like
Ag2cu(s) + Hno3(Aq) > Ag(aq) + Cu(aq) + HNO3 (aq)???

Ag(s) + 2HNO3 = AgNO3 + NO2(g) + H2O
Cu(s) + 4HNO3 = Cu(NO3)2 + 2NO2(g) + 2H2O

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

Just add an excess of concentrated HNO3, even may be diluted, but reaction proceeds slower. The excess means in this case about 1.5 to 2 ml of concentrated HNO3 per 1 gram of an alloy.

This wouldn't happen to be an extra credit problem given in chem lab at the University of Texas, would it? Because I definitely have the same one. Thanks for the help guys!

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
i don't see how you get AgCl if you said Ag2Cu does not react with NaCl?

Ag(s) + 2HNO3 = AgNO3 + NO2(g) + H2O
Cu(s) + 4HNO3 = Cu(NO3)2 + 2NO2(g) + 2H2O
basically we are dissolving the alloy in the concentrated nitric acid
precipitating it and drying it
weighing it
and based on these equations
we find mass of Ag and Cu?
then find the mass percent composition of both?
i still don't see how (AgCl) was involved.

AgNO_{3}(aq) + NaCl(aq) > NaNO_{3}(aq) + AgCl(s)
or, in net ionic form
Ag^{+} + Cl^{} > AgCl(s)
NaCl is a source of Cl^{} that will precipitate AgCl.

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
By calculating this,
i would get the % composition of the silver?
Like so (mass of Ag/mass of AgCl x 100) ?

By calculating this,
i would get the % composition of the silver?
Like so (mass of Ag/mass of AgCl x 100) ?
Exactly. You dissolve alloy, precipitate AgCl and filtrate it  at this moment you have solid that you can weight (after drying), mass of this solid allows calculation of mass of Ag  and bingo, everything is done.