Name________________________________
Dry Lab Exercise/Chem B1A Summer 2015
By Mike Daniel
Purpose – To determine the cation and anion gravimetrically in an alkali halide unknown identified as
MX. There are also some thermochemical questions. The cation could be any group 1 ion or ammonium
(NH4+) and the anion could be any halide (F-, Cl-, Br-, I-).
Due Date – Monday, July 20, 2015
Fill out the grey shaded blanks in this table and show your work in #1 and #2 following this table. Answer
the statements following the table. Show all work and answer questions with complete sentences.
Procedure
Part 1
1. A buret was rinsed with 0.101 M AgNO3 and
filled.
2. Add enough silver nitrate solution to a buret to
deliever about 10 mL to a weighed test tube.
3. Final buret Volume of AgNO3
4. Initial buret Volume
5. Mass of test tube 1
6. Volume of AgNO3 added to test tube 1
Observations
7. Mass of unknown MX
8. Unknown MX was dissolved in about 2 mL of
distilled water.
0.207 g
MX was a white crystalline solid with crystals a
little smaller than sugar. They quickly dissolved in
water. The aqueous solution was colorless and
homogeneous.
A thick white cloud was produced as MX was
added. Mixing spread the cloud throughout the
liquid. Centrifuging resulted in a white solid
precipitate (PPT) at the bottom and a colorless
liquid above the solid.
This produced a white PPT. Centrifuging and
further addition of MX(aq) eventually did not
produce a white PPT. This indicates the MX was in
excess.
Some white solid floating on the top of the liquid
was lost decanting. A white solid powder was left
in the test tube.
9. Almost all of the x solution was added to the
AgNO3 solution. This was mixed with a vortex and
then centrifuged for several minutes.
10. A drop of MX(aq) was added to the AgNO3 .
More MX was added until no PPT was observed.
11. The supernatant was decanted, a couple of mL
of distilled water was added, mixed, and
centrifuged. This washing was repeated a second
time.
12. The test tube was heated in an oven from 3:48
pm Wednesday afternoon to 1:40 pm the
following Monday. After cooling the test tube was
weighed.
13. Test tube 1 + AgX solid mass
Dry_Lab_Gravimetric_MX_from_AgX_v5.doc
AgNO3 is a colorless, homogeneous solution.
10.66 mL
0.59 mL
5.011 g
The solid had a light grey color. After weighing, the
solid was broken up with a spatula and the grey
color was only on the surface. The inside of the
solid was white.
5.250 g
Page 1
14. Mass AgX
15. Moles of AgNO3 (the limiting
reactant)=VM(AgNO3)= moles of AgX
16. Molar Mass of AgX
17. Molar Mass of X (subtract the molar mass of
silver from AgX)
18. Identity of X
19. Percent Error
Part 2
20. MX was weighed and added to test tube 2.
21. Test tube 2 mass
22. Test tube 2 + MX
23. Mass of MX
24. About 2 mL of DI water was added to the MX.
25. About half a gram of AgNO3 was weighed and
water was added to dissolve.
26. Most of the silver nitrate was added to the MX
solution.
27. After mixing and centrifuging, a drop of silver
nitrate was added to the MX test tube.
28. The supernatant was decanted, and the solid
was washed twice following steps 11 & 12. The
heating was from Monday at 4:01 pm to
Wednesday at 1:59 pm.
29. Dried test tube 2 + product from AgNO3 + MX
30. Mass of AgX
5.810 g
5.953 g
homogeneous, colorless solution
0.599 g
homogeneous, colorless solution.
A white cloudy mixture was made.
The liquids were seen to mix but no ppt was made,
indicating MX was LR.
The dried solid was a light grey color.
6.012 g
31. Moles AgX (use identity of X from part 1)=
32. Molar mass of MX = g MX / mol MX
33. Identity of M (subtract the accepted value of
the molar mass of X from MX)
34. Percent Error
Dry_Lab_Gravimetric_MX_from_AgX_v5.doc
Page 2
1. Label and show your calculations for steps 14-19 and box the answer below used to fill in the blanks
above. For example, show #14 = #13 - #5 = 5.250 g – 5.011 g = 0.239 g
2. Label and show your calculations for steps 30-34 and box the answer used to fill in the blanks above.
3. Identify the cation and anion experimentally determined. How would loss of precipitate in both parts
1 and 2 affect the two molar mass calculated?
Dry_Lab_Gravimetric_MX_from_AgX_v5.doc
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4. How would having water present in your precipitate affect the molar mass calculated in parts 1 and
2?
5. Calculate the enthalpy change when one mole of solid ammonium nitrate dissolves in water. (ΔH°f
NH4NO3 = -366 kJ/mol)
6. Draw an enthalpy diagram showing the above reaction.
Dry_Lab_Gravimetric_MX_from_AgX_v5.doc
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7. Would this reaction cause the aqueous solution to get colder or hotter? Explain
8. The ∆H° calculated in #5 is the enthalpy change when one mole of ammonium nitrate dissolves in
water. Calculate how 20.0 g of ammonium nitrate would change the temperature of 50.0 g of water. The
specific heat of this aqueous solution is 3.88 J/g °C.
q = s m ∆T
The heat removed/added to water will be what you calculate for heat from 20.0 g of ammonium nitrate
and the mass of solution will be the combined mass of water and ammonium nitrate.
Dry_Lab_Gravimetric_MX_from_AgX_v5.doc
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