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Hydrate Lab: Percent Composition of an unknown Hydrate

Last updated about 2 years ago
16 questions
Introduction:

Hydrate, any compound containing water in the form of H2O molecules, usually, but not always, with a definite content of water by mass. The best-known hydrates are crystalline solids that lose their fundamental structures upon removal of the bound water. Many ionic compounds, when crystallized from a water solution, take up a definite proportion of water as a part of their structure. The compound containing this water is known as a hydrate and the water is known as the water of hydration. If the hydrate is strongly heated, this water of hydration can be “driven off” as a vapor. The solid remaining is known as the anhydrous salt.

Substances that spontaneously absorb water from the air to form hydrates are known as hygroscopic or deliquescent, whereas hydrates that lose so-called water of hydration or water of crystallization to form the anhydrous substances are known as efflorescent. In many cases, the uptake and loss of water (by heating, decreasing pressure, or other means) are reversible processes, sometimes accompanied by changes in color. For example, copper II sulfate pentahydrate (CuSO4∙5H2O), is blue, copper II sulfate trihydrate (CuSO4∙3H2O) is blue, and anhydrous copper II sulfate (CuSO4) is white/grey.

Mass of water = Mass of hydrate – Mass of anhydrous salt or (Mass of water = Mass of compound before heating – Mass of compound after last heating)

The percentage of water in the hydrate can easily be calculated using the formula:

% Water = Mass of water x 100
Mass of hydrate

Formulas of Hydrates: Hydrates can contain varying amounts of water molecules in their structures. Some ionic compounds may have one, two, three, or more molecules of water associated with them. A common hydrate involves copper II sulfate. If there are five molecules of water associated with the compound copper II sulfate. The formula for this hydrate is CuSO4 ·5H2O. The name given to CuSO4 ·5H2O is copper (II) sulfate pentahydrate.
Objective:
  1. To determine the percentage of water in a hydrate and determine the number of water molecules in each hydrate.
  2. To determine the empirical formula of the hydrate and name each.
Materials:
Hydrate, porcelain crucible, crucible tongs, ring stand, iron ring, pipe stem triangle, wire gauze, Bunsen burner set-up, and electronic balance.

Procedure:
1. Set up the heating apparatus as demonstrated.
2. Mass the empty crucible. Record observations.
3. Mass between 1.00 g and 1.25 g of hydrate. Record observations.
4. Heat the crucible and its contents gently for 2 minutes and then more strongly for 3 minutes longer.
5. Mass the crucible and its contents and record the mass of the 1stheating.
6. Transfer the crucible back to the heating apparatus and heat strongly for 3 minutes.
7. Mass the crucible and its contents and record the mass of the 2nd heating.
8. If the difference in mass between 1st and 2nd heating is greater than 0.03 grams, repeat steps 7-9 to obtain a mass of the 3rd heating.
9. After you have completed the data collection, use a dropper to add water back into the anhydrous salt. What do you observe? Is the reaction endothermic or exothermic?
10. Rinse your product into the waste beaker. Rinse thoroughly, then heat with the Bunsen Burner.
Required
2

Data Table: input data, including what your unknown sample was

Required
1

Determine the mass, in grams, of the hydrate. SHOW ALL CALCULATIONS AND UNITS. Circle answer.

Required
1

Determine the mass, in grams, of anhydrous salt. SHOW ALL CALCULATIONS AND UNITS. Circle answer.

Required
1

Determine the mass, in grams, of water. SHOW ALL CALCULATIONS AND UNITS. Circle answer.

Required
1

Determine the mass percentage of water in the hydrate. SHOW ALL CALCULATIONS AND UNITS. Circle answer.

Required
1

Convert the grams of water into moles of water. SHOW ALL CALCULATIONS AND UNITS. Circle answer.
(1 mole H2O = 18.01 g H2O)

Required
1

Convert the grams of anhydrous salt into moles of anhydrous salt. SHOW ALL CALCULATIONS AND UNITS. Circle answer.
(1 mole CuSO4 = 159.61 g CuSO4)

Required
1

What is the mole ratio of water to anhydrous salt? SHOW ALL CALCULATIONS AND UNITS. Circle answer.
(Divide moles of water by moles of CuSO4)

Required
1

What is x (the number of water molecules bonded to CuSO4)?

Required
1

What is the formula of the hydrate?

Required
1

What is the name of the hydrate?

Required
0.5

What is a hydrate?

Required
0.5

What is a hydrate called after the water has been removed?

Required
0.5

Substances that spontaneously absorb water from the air to form hydrates are known as?

Required
0.5

Why is it important that the crucible be cooled to room temperature before massing on an electronic balance?

Required
1

What was the purpose of the second and possibly the third heating during the experiment?