Lab 20: (Hydrates and Solution Concentration) Name
Purpose:
Part 1: To determine the water of hydration in CuSO4• ? H2O and complete the chemical formula for the compound. Part 2: To dissolve the compound in water and determine its solution concentration.
Background:
The water in a crystal is called the water of hydration or water of crystallization.
A compound that contains water of hydration is called a hydrate.
Some substances, like cobalt(II) chloride change color when they gain or lose water of hydration.
Substance Formula Color
Cobalt(II) chloride (anhydrous) CoCl2 blue
Colbalt(II) chloride hexahydrate CoCl2•6H2O pink
It is possible to test for the presence of water in an experiment or dry environment by treating paper with anhydrous cobalt(II) chloride. The paper will be blue in a dry environment but if it comes in contact with water, it will convert to the hexahydrate and turn pink.
Substances that absorb moisture from the air are called hygroscopic and if they absorb enough moisture from the air to dissolve themselves they are called deliquescent. These substances can be used as chemical drying compounds called desiccants.
The concentration of a solution can be determined in several ways:
Percent by volume [%v/v] =
Percent (mass/volume) [%m/v] =
Note: Solute = substance getting dissolved Solvent = substance doing the dissolving
ü In this lab, you will determine the molarity and percent by mass of the solution you make.
Materials:
q Lab Apron
q Safety Goggles
q Plastic weighing tray
q Glass rod, Beaker, 50 mL graduated cylinder, and clear plastic cup
q Evaporating dish, watch glass, and crucible tongs
q Hot Plate, or (Burner, ring stand, iron ring, and clay triangle)
Chemicals:
q Copper(II) sulfate hydrate, CuSO4•?H2O
Safety Alert:
In this lab observe all precautions, especially the ones listed below and in the procedures.
q Protect your eyes! Wear your safety goggles with all steps.
o Copper(II) sulfate is a skin and respiratory irritant and moderately toxic by ingestion.
q Protect your lungs!
o Do not inhale the smoke or gases produced during a chemical reaction.
q Never taste chemicals!
o Treat all chemicals as toxic.
o Wash hands after handling chemicals.
q Protect our environment!
o Dispose of all materials according to the instructions of your teacher.
Procedures:(Safety goggles MUST be worn during all steps)
1. Obtain a sample of copper(II) sulfate hydrate from your teacher.
2. Measure and record the mass of a clean, dry, evaporating dish.
3. Place the copper(II) sulfate hydrate in the evaporating dish and determine the mass of the evaporating dish and hydrate.
4. Note the initial color of the copper sulfate hydrate.
5. Turn the hot plate on high heat.
6. Place the evaporating dish with the copper sulfate hydrate on the hot plate and heat on high for about ten minutes.
7. Turn off the burner. Cover the crucible with a watch glass and allow the sample to cool for about ten minutes. Note the color of the crystals.
8. After the evaporating dish has cooled for 10 minutes and is cool enough to handle safely, measure and record the mass of the evaporating dish and dehydrated sample.
9. Pour about 25 mL of distilled water onto the dehydrated copper sulfate in the evaporating dish and stir with the glass rod to dissolve the solid.
10. Transfer the solution to a 50.0 mL graduated cylinder. Rinse out the evaporating dish with additional distilled water in order to get any remaining residue into the cylinder. Continue to add distilled water until the volume of the solution is 50.0 mL.
11. Calculate the solution concentration in Molarity and Mass%. Show work in the calculation section.
12. Label a clear plastic cup with the Period, Lab station, and Date. Transfer the solution to the cup and give the cup to your teacher to let slowly evaporate.
13. Rinse and dry the graduated cylinder and evaporating dish. Clean up the lab station and wash your hands. / Data and Observations:
Mass of
Evaporating dish 52.83 g
Mass of
Evaporating dish
and hydrate: 59.07 g
Initial color
of hydrate Blue
Color of hydrate
after heating for
10 minutes Pale
grayish-green
Mass of
Evaporating dish and
dehydrated sample 56.82 g
Lab 20: (Hydrates and Solution Concentration) Name
Calculations (Show your work)
1. Using the data you collected in procedure 2 and 3:
a. Calculate the mass of the hydrated sample CuSO4• _??_ H2O
2. Using the data you collected in procedure 2 and 7:
a. Calculate the mass of the dehydrated sample of CuSO4.
b. Using the mass you calculated in 2a, determine the moles of CuSO4 in the dehydrated sample. (Use unit-conversions and consult periodic table to determine the molar mass)
3. Use your results from Question 1 and Question 2a above:
a. Calculate the mass of water in the original hydrated sample.
b. Using the mass you calculated in 3a, determine the moles of H2O in the original sample.
(Use unit-conversions and consult periodic table to determine the molar mass)
4. Determine the amount of water in the formula by comparing the moles of H2O to the moles of CuSO4.
Your answer will determine the water portion of the formula CuSO4• ___ H2O
5. Calculate the solution concentration for the liquid you placed into the clear plastic cup:
a. Calculate the Molarity of the solution you prepared.
b. Calculate the percent (mass/volume) of the solution you prepared.
Percent (mass/volume) [%m/v] =
Conclusion /Analysis Questions:
1. Complete the chemical formula for the original hydrated sample. (See answer #4 from calc. section)
CuSO4•____ H2O
- Restate the concentration (in mol/L) of the solution you made in the lab today.
- What would the concentration (in mol/L) be if you had added enough distilled water to make 100 mL of copper(II) sulfate solution instead of 50 mL of copper(II) sulfate solution using the same mass of dehydrated CuSO4?
- Describe how you could make 500.0 mL of a 0.50 M solution of copper(II) sulfate.