Title
Gravimetric Analysis of the Percent Silver in an Alloy
AP Chemistry
Students
West Lafayette HS
Mr. von Werder
Percy Silver (Primary on this lab report)
Alan Loy
Dates
September 4, 2013
Pre-lab discussion. Began procedure (see P1-P3 below).
~ 30 min.
September 5, 2013
Continue procedure (see P4-P5 below)
~ 45 min.
September 6, 2013
Procedure completed (see P6 below). Post-lab discussion.
~ 30 min.
Purpose
The purpose of the experiment is to determine the percentage of silver in a copper/silver alloy using
gravimetric analysis.
Synopsis
The silver/copper alloy is reacted with nitric acid.
Sodium chloride is added to precipitate the silver ions.
The precipitate is filtered, rinsed, dried, and then massed.
Safety
General safety guidelines are observed. Specific concerns related to the materials and procedures used:


Procedure
Disposal
Concentrated HNO3 needs to be handled with care.
The alloy/HNO3 reaction produces noxious gas (NO, NO2) and should be done in the fume hood.
P1
A sample of the silver alloy was massed directly into a 150 mL beaker.
P2
10 mL of concentrated HNO3 was added to the alloy (in the fume hood).
P3
An appropriate mass of solid NaCl was calculated (see A2 below), massed, dissolved in 25 mL of
distilled water and added to the alloy solution. The mixture was covered and allowed to sit
undisturbed overnight.
P4
Vacuum filtration with the Gooch crucible separates the settled AgCl precipitate. A previously
oven-dried Gooch crucible and filter paper is massed. The AgCl mixture is filtered and rinsed.
P5
The filter paper/precipitate is place in an evaporating dish in a low temperature oven to dry.
P6
The final mass of the filter pater/precipitate is measured.
The filtrate (see P4 above), containing mainly excess HNO3 and copper ions, was emptied to a common
waste container to later be neutralized with sodium bicarbonate by the teacher.
The silver alloy was scraped from the filter paper to a waster container for AgCl, to possibly dissolve and
reclaim the silver at a later date by the teacher.
Data
Qualitative Data

When the nitric acid was added to the alloy (see P1 above), a brown gas was produced (released into
the fume hood). The solution turned green.

When the sodium chloride solution was added to the alloy solution (see P3 above), a white precipitate
formed and the solution became a light blue.

In the filtration process (see P4 above), the white/grayish precipitate was retained in the Gooch
crucible. The filtrate was a clear blue solution.
Quantitative Data
Mass of silver alloy
0.4958 g
Mass of Gooch crucible/filter paper
16.8753 g
Mass of Gooch crucible/filter paper/AgCl precipitate
17.2562 g
Analysis
A1
Balanced chemical reactions that were part of the process.
Reaction of copper with nitric acid, and silver with nitric acid (see P2 above).
Cu
+ HNO3
Ag
+ HNO3

Cu(NO3)2 + H2O

AgNO3 + H2O
+ NO2
+ NO2
Reaction of silver ions (alloy solution) with chloride ions (NaCl solution) (See P3 above).
Ag+(aq)
A2
 AgCl(s)
Cl-(aq)
+
Determine the number of grams of sodium chloride needed to precipitate the silver ions.
(Assuming the entire mass of the alloy is silver for this calculation).
0.4958𝑔 𝐴𝑔 ×
1 𝑚𝑜𝑙 𝐴𝑔
107.87 𝑔 𝐴𝑔
×
1 𝑚𝑜𝑙 𝐴𝑔+
1 𝑚𝑜𝑙 𝐴𝑔
×
1 𝑚𝑜𝑙 𝐶𝑙−
1 𝑚𝑜𝑙 𝐴𝑔+
×
1 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙
1 𝑚𝑜𝑙 𝐶𝑙 −
×
58.44 𝑔 𝑁𝑎𝐶𝑙
1 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙
This amount times two will ensure silver ions are the limiting reactant in an excess of chloride ions.
A3
-
- 16.8753 g
AgCl precipitate
= 0.3809 g
Determine the number grams of silver metal present from the mass of the AgCl precipitate.
0.3809 𝑔 𝐴𝑔𝐶𝑙 ×
1 𝑚𝑜𝑙 𝐴𝑔
143.32 𝑔 𝐴𝑔𝐶𝑙
×
1 𝑚𝑜𝑙 𝐴𝑔+
1 𝑚𝑜𝑙 𝐴𝑔𝐶𝑙
×
1 𝑚𝑜𝑙 𝐴𝑔
1 𝑚𝑜𝑙 𝐴𝑔+
×
107.87 𝑔 𝐴𝑔
1 𝑚𝑜𝑙 𝐴𝑔
= 0.2867 𝑔 𝐴𝑔
Determine the percentage silver in the alloy.
%𝐴𝑔 =
A6
17.2562 g
Mass of Gooch crucible/filter paper
= Mass of
A5
(0.5372 g NaCl)
Determine the number of grams of AgCl precipitate.
Mass of Gooch crucible/filter paper/AgCl precipitate
A4
= 0.2686 𝑔 𝑁𝑎𝐶𝑙
𝑚𝑎𝑠𝑠 𝐴𝑔
𝑚𝑎𝑠𝑠 𝑎𝑙𝑙𝑜𝑦
× 100
%𝐴𝑔 =
0.2867 𝑔 𝐴𝑔
0.4958 𝑔 𝑎𝑙𝑙𝑜𝑦
× 100 = 57.83 %𝐴𝑔
Determine the percent error. Actual values provided by the instructor.
%𝑒𝑟𝑟𝑜𝑟 =
𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙−𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑
%𝑒𝑟𝑟𝑜𝑟 =
𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑
× 100
57.83 %𝐴𝑔 − 57.68 %𝐴𝑔
× 100 = 0.26 %𝑒𝑟𝑟𝑜𝑟
57.68 %𝐴𝑔
Conclusion
The alloy was experimentally determined to contain 57.83 % silver, with a 0.26 % error.
Theory
Gravimetric analysis was the analytical method used. This method usually involves the dissolving of a
sample or chemically reacting a sample so that it is in solution form. A reagent is then added to cause
precipitation or crystallization of a substance from solution. The solid formed can now be filtered, rinsed,
dried and massed. Knowledge of the chemical reactions involved and accurate stoichiometry calculations
allow for the determination of amount of a particular substance present in the original sample.
Reflection
Based on the low percent error, the experiment went well. Steps most critical in assuring minimal error:



Transfer of the alloy to the beaker (this is why the alloy was massed directly into the beaker ).
Transfer of the precipitate to the filter crucible (rubber policeman was used to help with the clean-up).
Heating to remove water from the crucible/precipitate (why it was heated overnight).