ChemistryChapter 6

DETERMINATION OF THE FORMULA OF COPPER (II) SULFATE

Purpose

To determine the number of moles of water in a mole of hydrated copper (II) sulfate is written as CuSO4•n H2O.

Introduction

Vocabulary: Salt, anhydrous salt, hydrated salt, crystal

A hydrate is a compound that contains water in its crystal structure. The water may be removed from the salt in the laboratory by heating the salt. The salt without the water is called an anhydrous salt. Here are some examples of hydrated salts:

CaSO4 · 2H2O CoCl2 6 H2O MgSO4 · 7H2O

In a hydrate the water molecules are a distinct part of the compound but are joined to the salt by connections that are weaker than the connections in the salt or the connections in the water molecules. Notice we use a dot to connect the water units to the salt formula.

In this experiment you will measure the mass of copper(II) sulfate, then remove the water from the crystals and measure the mass of the anhydrous salt. The data gathered will allow you to determine the percent water in the hydrated salt and also the empirical formula for the hydrated salt (McDougal Litel 2007).

Prelab question—see page 3

Materials

Bunsen burner, hydrated copper (II) sulfate, test tube, test tube holders

Procedure

  1. Weigh a clean dry test tube.
  2. Put about 1cm depth of copper (II) sulfate in the test tube and reweigh.
  3. Record these weightings and those which follow in a table.
  4. Now, use the Bunsen to heat the test tube gently for a minute and then strongly for about 3 minutes. Hold the test tube with a test tube holder. Allow the tube to cool and then reweigh it.
  5. Heat the test tube again for about 2 minutes. Allow it to cool and then reweigh.
  6. If the last two readings differ by more than 0.05g, repeat the heating and weighing again.

Data

Observations: Record all observations including appearance of the salt and what happens during heating.

Record your measurements with uncertainty and units. Give this table a title.

Mass of test tube:
Mass of test tube and hydrated copper (II) sulfate:
Mass of test tube and anhydrous copper (II) suflate:

Calculations: Show your work, and then write them in a table. Give this table a title, and include units in calculations.

Mass of anhydrous copper (II) sulfate:
Moles of anhydrous copper (II) sulfate:
Mass of H2O:
Moles of H2O:
Mole ratio of H2O and CuSO4 = Moles H2O / moles CuSO4
Calculated empirical formula of CuSO4
Literature value of moles H2O
Percent error

*Mass of hydrated copper (II) sulfate: = (Mass of test tube and hydrated copper (II) sulfate – mass of test tube)

Conclusions

  1. Why is the salt heated again if you don’t get the same mass?
  2. Write a conclusion and evaluate the experiment.
  3. What are some sources of error for the experiment? If you made a mistake or had bad lab technique, this is called “human error.” Think of other sources of error, such as weaknesses in the procedure and with the equipment

Evaluations

If you had less than 10% error, then your lab technique was probably good. The rest of the error is due to “random error” caused by limitations of the procedure and the equipment.

  1. How was your lab technique? Do you think you need to improve your technique?
  2. How could you improve the procedure?
  3. Was the equipment accurate enough? Do you think you need better lab equipment?

Prelaboratory Question

Prelab question: The following data was collected in an experiment. Calculate the formula of the hydrated salt.

Table 1: Mass in grams of heating hydrated copper (II) chloride

Mass of test tube: / 22.88 g ± 0.01 g
Mass of test tube and hydrated copper (II) chloride: / 24.17g ± 0.01 g
Mass of test tube and anhydrous copper (II) chloride: / 23.90 g ± 0.01 g
  1. Mass of anhydrous copper (II) chloride:
  1. Moles of anhydrous copper (II) chloride:
  1. Mass of H2O:
  1. Moles of H2O:
  1. Mole ratio of H2O and CuCl2= Moles H2O / moles CuCl2
  1. Calculated empirical formula of CuCl2
  1. Literature value of moles H2O
  1. Percent error