Name: ______Per.______Date:______

Formula of a Hydrate Lab

The purpose of this lab is to determine the percent by mass of water in hydrated copper(II) sulfate, and to establish the formula for the salt. Many salts occurring in nature (or even purchased from chemical suppliers!) are hydrated; that is, a number of water molecules are bound to the ions in the crystalline structure of the salt. For many of these hydrated salts, such as Epsom salt, heat removes the water molecules: MgSO4 • 7 H2O ------> MgSO4 (s) + 7 H2O (g)

Procedure:

1. Record the mass of evaporating dish. Add 2-3 g of copper(II) sulfate (X)-hydrate to the dish. Record the total mass.

2. Place the dish on the hot plate: Heat the dish gently to remove the water, then gradually intensify the heat over a few minutes. Do not allow the dish to become red hot. Continue to heat for 12 minutes. Carefully use tongs to remove the dish from the hot plate. Set on lab bench to cool for 5 minutes.

3. Reweigh the dish + salt.

4. Add back a few drops of water and observe and changes.

5. The anhydrous copper(II) sulfate must be placed in the waste container.

Wash the dish & clean-up your lab area.

Calculations:

1. Determine the mass of the anhydrous salt, hydrated salt and water lost

2. Determine the percent of mass by water of in the hydrated salt.

3. Determine the formula of the unknown hydrate.

Questions

1. A student heats a salt weighing 3.40 grams. After heating, it weights 3.05 grams. Could this salt be a hydrate? What evidence supports your answer? Talk chemistry

2. When you heated the salt in the lab, was this a physical change or a chemical change? What evidence supports your answer?

3. A student heats 4.20 g of nickel(II) acetate X hydrate [ Ni(C2H3O2)2 • X H20 ]. After heating, the anhydrous salt weighs 2.98 g. What is the formula for the hydrate?

4. A student heats 4.35 g of calcium sulfate X hydrate. After heating, the anhydrous salt weighs 3.44 g. What is the formula for the hydrate?

Error Analysis

Determine if the following errors would make the mole-ratio of water to anhydrous salt come out

too <High, too <Low, or <No Effect...

____ the hot plate placed black soot on the bottom of the crucible

____ the copper(II) sulfate wasn't completely anhydrous after heating

____ some of the salt splattered out during heating while you weren't watching

____ your dish weighed more than everyone else's

____ you incorrectly thought the formula for copper(II) sulfate was Cu2SO4

Practice AP Lab Question

The following is an actual AP Chemistry exam question. Answer it as part of your lab write-up. An experiment is performed to determine the empirical formula of a copper iodide formed by direct combination of elements. A clean strip of copper metal is weighed accurately. It is suspended in a test tube containing iodine vapor generated by heating solid iodine. A white compound forms on the strip of copper, coating it uniformly. The strip with the adhering compound is weighed. Finally the compound is washed completely from the surface of the metal and the clean strip is dried and reweighed.

DATA TABLE
Mass of clean copper strip 1.2789 grams
Mass of copper strip and compound 1.2874 grams

Mass of copper strip after washing 1.2748 grams

  1. State how you would use the data above to determine each of the following. (Calculations not required.)
  2. The number of moles of iodine that reacted
  3. The number of moles of copper that reacted

b.  Explain how you would determine the empirical formula for the copper iodide.

  1. Explain how each of the following would affect the empirical formula that could be calculated.
  2. Some unreacted iodine condensed on the strip.
  3. A small amount of the white compound flaked off before weighing.
  4. The strip was not thoroughly dry for the last massing.
  5. When drying the strip, it was heated too strongly, resulting in some dark copper oxide formation on the surface.