Experiment 8 - Analysis of a Hydrate

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Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Experiment 8 – Analysis of a Hydrate
Introduction
When most people think of a solid substance, it almost never crosses their minds that it could be
anything but completely dry (on the inside, that is). However, there are many solids that
naturally incorporate water into their solid matrix when they form.. These compounds are called
hydrates and are hygroscopic, or water absorbing. Each of these hydrates contains a certain
number of water molecules, and it follows the law of constant composition, as do all compounds.
The way the waters are notated, however, looks slightly differently.
For instance, copper (II) sulfate, CuSO4, incorporates 5 waters when it forms a solid. Its
chemical formula is written as CuSO4·5H2O, where the “dot” refers to the fact that the water is
not part of the chemical structure, but merely absorbed in the solid. These hydrates can be
heated, and the water driven off from the solid. In this case the anhydrous, or “without water”,
dry, form of the solid is obtained.
Alum is the general name for potassium aluminum sulfate dodecahydrate, or simply,
KAl(SO4)2·12H2O. It is used commonly in deodorants and as a styptic pencil to stop bleeding
from minor cuts. When it is heated, and dried, it follows the chemical equation:

𝐾𝐴𝑙(𝑆𝑂4 )2 · 12𝐻2 𝑂 (𝑠) → 𝐾𝐴𝑙(𝑆𝑂4 )2 (𝑠) + 12 𝐻2 𝑂 (𝑔)
In this way, the percentage of water in a sample can be found. Theoretically, the percentage in a
hydrate is merely the molar mass of water divided by the molar mass of the compound.
Experimentally the amount of water in any hydrate can be determined by comparing the mass of
the hydrate before and after heating.
In this experiment you will determine, experimentally, the percentage of water in alum and
compare that to the theoretical value. After this you will experimentally determine the
percentage of water in an unknown hydrate, as well as the water of crystallization: the number
of waters in the hydrate.
Equipment
100 or 250 mL beaker
Watchglass
Bunsen burner
Wire gauze
Iron ring
NaHCO3
Unknown sample
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Examples
1) Calculate the theoretical percentage of water in alum, KAl(SO4)2·12H2O.
First calculate the molar mass of water and of the hydrate as a whole.
Alum
1 K:
Water
𝑔
𝑔
𝑔
𝑔
𝑔
𝑔
1 x 39.10 𝑚𝑜𝑙 = 39.10 𝑚𝑜𝑙
1 Al: 1 x 26.98 𝑚𝑜𝑙 = 26.98 𝑚𝑜𝑙
2 S:
8 O:
𝑔
𝑔
24 H: 24 x 1.01 𝑚𝑜𝑙 = 24.24 𝑚𝑜𝑙
𝑔
𝑔
12 O: 12 x 16.00 𝑚𝑜𝑙 = 192.00 𝑚𝑜𝑙
2 x 32.07 𝑚𝑜𝑙 = 64.14 𝑚𝑜𝑙
___________
8 x 16.00
216.24 𝑚𝑜𝑙
𝑔
𝑚𝑜𝑙
𝑔
𝑔
= 128.00 𝑚𝑜𝑙
____________
𝑔
258.22 𝑚𝑜𝑙
𝑔
Total mass of alum = 474.46 𝑚𝑜𝑙
Calculate the percentage of water in the hydrate:
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
% 𝐻2 𝑂 = 𝑚𝑎𝑠𝑠 𝑜𝑓 ℎ𝑦𝑑𝑟𝑎𝑡𝑒 𝑥 100%
𝑔
𝑚𝑜𝑙
% 𝐻2 𝑂 =
𝑔 𝑥 100% = 𝟒𝟓. 𝟓𝟖% 𝑯𝟐 𝑶
474.46
𝑚𝑜𝑙
216.24
2) An unknown hydrate is weighed out, heated, and reweighed after drying. If the original
hydrate weighed 1.725 g, but after heating it weighed 0.941 g, what is the percentage of water in
the unknown compound?
Using Conservation of Matter, the amount of water that is lost is the difference between
before and after heating
1.725 g hydrate – 0.941 g anhydrous = 0.748 g water lost.
0.745 𝑔
% 𝐻2 𝑂 =
𝑥 100% = 43.188% 𝐻2 𝑂 → 𝟒𝟑. 𝟐% 𝑯𝟐 𝑶
1.725 𝑔
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
𝑔
3) If an anhydrous compound has a molar mass of 245 𝑚𝑜𝑙 and it was found that the % H2O was
30.6%, calculate the water of crystallization of the compound.
Here it is useful to remember that percentages do not depend on the amount of the
sample. If you assume that there were 100g of the original sample (even if it were less),
then the percentages become masses (30.6% of 100g is 30.6 g). The result is 100 g of
hydrate minus 30.6 g water yields 69.4 g anhydrous compound.
Taking the masses of the compounds and dividing by the molar masses of the compounds
gives the number of moles. These, in turn, can be turned into the ratio of water to anhydrous
compound as follows:
1 𝑚𝑜𝑙 𝐻 𝑂
30.6 g 𝐻2 O [18.02 𝑔 𝐻2 𝑂] = 1.70 𝑚𝑜𝑙 𝐻2 𝑂
2
1 𝑚𝑜𝑙 𝑎𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠
69.4 g anhydrous [ 245 𝑔 𝑎𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 ] = 0.283 𝑚𝑜𝑙 𝑎𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠
1.70 𝑚𝑜𝑙 𝐻2 𝑂
0.283 𝑚𝑜𝑙 𝑎𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠
= 6.01 ≈ 6
Therefore, the water of crystallization is a 6:1 ratio, water:anhydrous. The formula of the
hydrate is:
Anhydrous Compound·6H2O
Figure 8.1: Experimental Set Up
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Procedure
Percentage of Water in Alum
Weigh between 0.8 – 1.2 g of alum into a clean, dry 250 mL beaker.
Place the watchglass on the beaker. This will act as the indicator of when the experiment is
done.
Place the beaker on the wire gauze and heat the alum. A vigorous flame will be used in this
experiment. Be sure that the tip of the cone of the flame is touching the wire gauze. See
Figure 8.1. Water droplets should begin to form on the watchglass within a few minutes.
Note: the initial “fog” one sees upon heating is merely the water vapor that is naturally
occurring on the glass being driven off by the heat.
Continue to heat the alum until the water droplets on the watchglass have disappeared.
Allow the beaker to cool to room temperature and weigh it.
anhydrous alum resulting from the decomposition.
This is the mass of the
Calculate the percent of water of the alum from the original mass of the alum.
Percent of water in an Unknown Hydrate
Weigh out approximately 1 to 2 g of the unknown sample into your beaker.
Repeat the steps of the previous procedure.
Calculate the average percentage of water in the unknown hydrate.
Calculate the water of crystallization of the compound and report its chemical formula.
Post Experiment Instructions
All waste can be placed in the trash or rinsed down the sink.
Return unknown vial to its place of origin.
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Prelaboratory Questions
1) What is meant by the following terms:
Hydrate
Anhydrous
Water of crystallization
2) What is the %H2O in a sample of CaCl2·2H2O?
3) An unknown sample of a hydrate was dehydrated and found to contain 13.7% water. If the
𝑔
molar mass of the anhydrous compound is 227 𝑚𝑜𝑙 what is the chemical formula of the hydrate?
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Data Table
Percent of H2O in KAl(SO4)2·12H2O
Trial 1
Trial 2
Mass of KAl(SO4)2·12H2O
(before heating)
__________
__________
Mass of KAl(SO4)2·12H2O
(after heating)
__________
__________
Mass of H2O
__________
__________
Percentage of water in the hydrate – show calculations
Percent of H2O in KAl(SO4)2·12H2O
__________
Average percent H2O in KAl(SO4)2·12H2O
__________
__________
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Percent of Water in an Unknown Hydrate
Unknown Number ____
Trial 1
Trial 2
Mass of Unknown Hydrate
Before Heating
__________
__________
Mass of Unknown Hydrate
After Heating
__________
__________
Mass of H2O
__________
__________
Percentage of H2O in the Unknown Hydrate – show calculations
% H2O in Unknown Hydrate
__________
__________
Average %H2O in the Unknown Hydrate __________
𝑔
Water of Crystallization of the Unknown Hydrate – Molar Mass Unknown _____________𝑚𝑜𝑙
Water of crystallization
__________
Chemical Formula of the Hydrate
____________
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
Postlaboratory Questions
1) Calculate the percentage of water in the following compounds
CoCl2·6H2O
CuSO4·5H2O
2) An unknown hydrate sample has a mass of 1.026 g. It is heated until it is dry and it weighs
0.656 g after heating. What is the %H2O of the sample?
3) If the molar mass of the anhydrous compound in problem 2 were 320 g/mol, what is the
water of crystallization, and chemical formula of the hydrate?
Experiment 8 – Analysis of a Hydrate
Name __________________
Lab Section __________________
4) A sample of a hydrate of CoCl2 is found to have 13.88%H2O. What is the water of
crystallization and chemical formula for this hydrate?
5) Gypsum is a naturally occurring hydrate of CaSO4. If it a sample is dehydrated and it is
found to contain 21.0% water, what is the chemical formula for gypsum?
6) Alternatively to the above, Plaster of Paris, the mortar like compound used extensively in
artwork such as frescos, is a fractional hydrate. This means that its overall chemical formula is
not a whole number. If a 1.401 g sample of Plaster of Paris is dried and its resulting mass is
1.315 g, what is the formula for Plaster of Paris?
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