FORMULA OF A HYDRATE
Some salts, when crystallized from water solutions, retain definite proportions of water as an integral part of the crystal structure. This type of crystal is called a hydrate. In this experiment, you will determine the whole number proportion of water to salt in a hydrate. This will enable you to write the correct formula for a hydrate (salt · xH2O).
OBJECTIVES
When you have completed this activity, you should be able to:
1. Demonstrate a procedure for determining the amount of water and salt in a hydrate.
2. Compute the smallest whole number ratio of moles of water to moles of salt in a hydrated salt sample.
3. Construct a formula for a hydrate from the whole number ratio of moles of water to moles of salt.
4. Perform an error analysis.
5. Construct a definition for hydrated salts and anhydrous salts.
MATERIALS
· goggles
· crucible tongs (substitute hot pads to protect your hands from heat)
· cooling rack
· balance, sensitive to at least 0.01 g
· small baking dish/ ramekin and cover (substitute any kind of baking dish that can go into the oven)
· burner (substitute an oven to dry your baking dish and its contents)
· Epsom salts (magnesium sulfate hydrate)
PROCEDURE
1. Dry the ramekin or baking dish and cover in an oven overnight. Record the mass of the empty, dry dish and cover in Data Table entry #4.
OR
Place a clean, dry baking dish or ramekin with the cover ajar on the burner into the oven and heat to redness for 10 minutes at 160◦F. Discontinue heating and carefully use tongs hot pads to set the remove the dish and cover on a onto a cooling rack to cool. When cool enough to transport (about 5 minutes), measure the mass of the dish and cover. Record the mass of the empty, dry dish and cover in Data Table entry #2. Repeat the heating, cooling, and measuring procedure until the mass is constant. You have obtained a constant mass when two consecutive masses differ by no more than 0.01 g. Record the mass of the empty, dry dish and cover in Data Table entry #3 and #4.
2. Half-fill the dish with about 3.0 g of Epsom salts (magnesium sulfate hydrate). Measure and record the exact mass in Data Table entry #1.
3. Determine the mass of the hydrate by subtracting entry #4 from entry #1 in the Data Table. Record this result in Data Table entry #5.
4. Gently heat (250∘F) the dish and contents with the cover ajar for two minutes. Gradually increase the temperature and heat strongly (about 400∘F) for 10 minutes. Cool, measure, and record the mass in Data Table entry #6.
5. Repeat the strong heating with cover ajar for two minutes. Cool, measure the mass, and record reading in Data Table entry #7. If a third heating is necessary, heat the dish and its contents for one additional minute. Record the mass in the margin next to entry #7. Repeat the heating, cooling, and measuring process until two consecutive masses differ by no more than 0.01 g. Record the constant mass of the dish, cover, and anhydrous salt in Data Table entry #8.
6. Determine the mass of the anhydrous salt by subtracting entry #4 from entry #8 in the Data Table and record in Data Table entry #9.
7. Determine the mass of water lost by subtracting entry #9 from entry #5 in the Data Table and record in Data Table entry #10.
DATA TABLE
1. Mass of ramekin, cover, and hydrated salt / g2. Mass of ramekin and cover (1st heating) / g
3. Mass of ramekin and cover (2nd heating) / g
4. Constant mass of ramekin and cover / g
5. Mass of hydrate / g
6. Mass of ramekin, cover, and anhydrous salt (1st heating) / g
7. Mass of ramekin, cover, and anhydrous salt (2nd heating) / g
8. Constant mass of ramekin, cover, and anhydrous salt / g
9. Mass of anhydrous salt / g
10. Mass of water lost / g
ANALYSIS
1. Calculate the number of moles of water lost. (Take the # of grams in #10 and convert to moles.)
2. Calculate the number of moles of anhydrous MgSO4 (molar mass 120.3 g). (Take #grams in #9 and convert to moles)
3. Calculate the smallest whole number ratio of moles of water to moles of anhydrous salt. (Divide the moles of water by the moles of anhydrous salt.)
4. From your calculated whole number ratio, write the probable formula for magnesium sulfate hydrate.
5. Perform an error analysis.
a. Calculate the percent error (Remember to calculate % error= |Experimental value-Theoretical value|/ Theoretical value X 100%). Compare the molar mass of your experimentally determined hydrate with the theoretical molar mass of the hydrate (246.7 g/mol)
b. List several possible sources of error. Include error inherent in the procedure itself.
6. Epsom salts are hydrates of magnesium sulfate. The formula for Epsom salt is MgSO4·7H2O. If a 7.834 g sample is heated until constant mass is obtained, what is the mass of the anhydrous magnesium sulfate? What percent of the hydrate is water?
7. A certain hydrate of potassium aluminum sulfate has the formula KAl(SO4)2·xH2O. When a sample weighing 5.459 g is heated to remove all the water, 2.583 g of anhydrous salt remains. What is the mass % of water in the hydrate? Determine the value of x.
PRE-LAB: FORMULA OF A HYDRATE
Read the introduction and the procedure of the lab activity, highlighting key information as you read. Answer the following questions.
1. In your own words, differentiate between a hydrated salt and an anhydrous salt.
2. Why is it important to heat the baking dish or ramekin and cover in step #1?
3. Why is it necessary to achieve constant mass? How will you know when constant mass is achieved?
4. Why should the ramekin be cooled before taking a mass reading?
5. How might each of the following affect the reliability of your results? Explain.
a) Increased humidity in the room.
b) Eliminating the preliminary heating, cooling, and massing of the ramekin and cover prior to its use.
c) Recording mass measurements to two significant figures.