Sample gravimetric analysis problems

1. GRAVIMETRY
Sample Problems

2. 𝐺𝐹 =
𝑎 𝑥 𝐹𝑊 𝑜𝑓 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 𝑠𝑜𝑢𝑔ℎ𝑡
𝑏 𝑥 𝐹𝑊 𝑜𝑓 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 𝑤𝑒𝑖𝑔ℎ𝑒𝑑
Where: a,b = small whole numbers that have values such that
the number of formula weights in the numerator
and denominator are chemically equivalent.
Gravimetric Factor, GF

3. Substance weighed Substance sought GF
AgCl Cl
BaSO4 S
Fe2O3 Fe
PbCrO4 Cr2O3
K2PtCl6 K
𝐹𝑊 𝐶𝑙
𝐹𝑊 𝐴𝑔𝐶𝑙
𝐹𝑊 𝑆
𝐹𝑊 𝐵𝑎𝑆𝑂4
𝐹𝑊 𝐶𝑟2 𝑂3
2 𝑥 𝐹𝑊 𝑃𝑏𝐶𝑟𝑂4
2 𝑥𝐹𝑊 𝐾
𝐹𝑊 𝐾2 𝑃𝑡𝐶𝑙6
2 𝑥 𝐹𝑊 𝐹𝑒
𝐹𝑊 𝐹𝑒2 𝑂3
Exercise!

4. A 1.000 g sample of ultrapure silicon
was treated with an excess Oxidizing
Agent and the resulting SiO2, after proper
filtration/collection and drying weighed
2.1387 g. Calculate % Si in the sample.
http://blog.teachersource.com/wp-content/uploads/2011/03/silicon11.jpg?w=150
http://niabraze.com/sites/default/files/computer_chips.png
https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcTmFH
TW1EohHAirss4oRIDRVWjO5BGoVCdiH0xWszxoR_qsKGaJIw
ANS. 99.97%

5. From the feldspar sample weighing 500.0 mg,
a mixture of sodium and potassium chlorides
was obtained weighing 118.0 mg. Subsequent
treatment with AgNO3 furnishes 245.1 mg of
AgCl. What are the percentages of Na2O and
K2O in the sample?
https://mineralseducationcoalition.org/wp-content/uploads/Feldspar2_353331161.jpg
ANS. 10.55 % K2O
3.66 % Na2O

6. A mixture containing AgCl and AgBr
weighs 0.4628 g. It is treated with Cl2 to
convert AgBr to AgCl. The sample now
weighs 0.3604 g. Calculate the %AgBr in
the sample.
https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcRZ
M9JvqlfjzIyKCa4kmFdaXu-2JFLD9dATX4f1CmueEc5PtUdyjQ
ANS. 93.47%

7. 2.364 g mixture of Na2SO4 and K2SO4
+ BaCl2
3.701 g BaSO4
FW: Na2SO4 142.05 g/mol
K2SO4 174.29 g/mol
BaSO4 233.37 g /mol
ANS. 74.5 % Na2SO4
25.5 % K2SO4

8. To determine the amount of iron in a
dietary supplement, a random sample of 15
tablets weighing a total of 20.5050 g was
ground into fine powder. A 3.1160 g was
dissolved and precipitated iron as Fe(OH)3.
The precipitate was collected, rinsed and
ignited to a constant weight as Fe2O3, yielding
0.3550 g. Report the iron content of the
supplement as g FeSO47H2O.
http://www.patriciamcconnell.com/theotherendoftheleash/wpc
ontent/uploads/2016/10/bigstock-Hardworker-6589658.jpg
https://www.meded101.com/wpcontent/uploads/2015/05
/Iron-Supplements-for-Anemia-Med-List-Review.jpg
ANS. 0.5423 g per tablet

9. A 0.3516 g sample of commercial
phosphate detergent was ignited at a red heat to
destroy the organic matter. The residue was taken
up in hot HCl, which converted the P to H3PO4.
the phosphate was precipitated as
MgNH4PO46H2O by addition of Mg2+ followed
by aqueous NH3. After being filtered and washed,
the precipitate was converted to Mg2P2O7
(FW=222.57) by ignition at 1000 C. This residue
weighed 0.2161 g. Calculate %P (FW=30.97) in
the sample.
http://www.slate.com/content/dam/slate/articles/business/branded/2011/12/111222_BRANDED_C
ascade.jpg.CROP.article568-large.jpg
ANS. 17.10 %