Question

Empirical Formula of Epsom Salt

Introduction

Hydrates are crystalline solids that contain a fixed number of water molecules as an integral part of their crystalline structure. The number of water molecules bound per metal ion is often characteristic of that particular metal ion. One of the common hydrates is hydrated magnesium sulfate (Epsom salt). Epsom salt is used to reduce inflammation when applied externally.

Epsom salts formula is sometimes written as MgSO₄.xH₂O. In this experiment, you will be trying to find the value of x (i.e. the number of moles of water associated with one mole of MgSO₄ in Epsom salts)

When Epsom salts are heated, their water of crystallization is driven off, leaving anhydrous magnesium sulphate, MgSO₄.

                        MgSO₄.xH₂O(s) →MgSO₄(s) + xH₂O(g)

Or generally

Hydrated salt   anhydrous salt + water vapour

Since many hydrates contain water in a stoichiometric quantity, it is possible to determine the molar ratio of water to salt.

By weighing the hydrated and anhydrous magnesium sulphate before and after heating it is possible to find the mass of MgSO₄ and H₂O in a sample of Epsom salts. From these masses you can calculate the moles of water associate with one mole of MgSO₄. You can then write the precise formula for Epsom salts

Procedure

Weigh a clean, dry boiling test tube with cork stopper

Add about 1cm depth of Epsom salts and reweigh.

Clamp the test tube to retort stand.

Heat the tube gently using Bunsen burner for 2 minute and then strongly for 8 minutes.

Turn off the Bunsen burner, stopper the tube and allow it to cool to room temperature and reweigh.

Heat the tube again for 5 minutes. Stopper the tube and allow it to cool to room temperature and reweigh. If the last two weights differ by more than 0.05g then repeat the heating and weighing until the weight remains the same for two consecutive weighing’s.

           

Data Sheet

1. Weight of test tube and stopper                                                              43.4421g    

2. Weight of stopper test tube and sample before heating                                48.5459g

3. Weight of sample                                                                                      5.1038

4. Weight of stopper test tube and sample after first heating                           46.5175

5. Weight of stopper test tube and sample after heating to constant weight     46.1879

                                        after second heating                                                46.1186

                                                                                                                        ----------

Treatment of results                                                                            0.0693      

Describe what happens as the Epsom salts are heated.

From your results table find:

The mass of hydrated MgSO₄ used.

The mass of water in the hydrated salt.

The mass of MgSO₄ in the hydrated salt

The percent of water in the hydrate. (Show calculations)

Moles of water in the hydrate sample (show calculations)

Moles of anhydrous salt in sample.

The relative molecular masses of H₂O and MgSO₄

Formula of anhydrous salt

Formula of the hydrated salt*. . . . . . . . . .. . .MgSO₄______H₂O

(Show calculations)

*The number of water molecules per anhydrous salt crystal = larger # of moles / smaller # of moles

Question

How could you show that it was water being given off when the Epsom salts are heated? Explain the test you would use and the results you would expect to see.

Why might you need to reheat the salt several times before getting the final mass of the salt?

What is the percent composition of oxygen in Al2(SO4)3? Show working.

Analysis of a sample of hydrated salt shows that it contains 4.00g Ca, 7.09g Cl, and 3.60g H₂O.

a)   What is the formula of the hydrated salt? Show working.

b)   Assume the salt sample released 4.5 grams of water. How many moles of water were in the sample? Show working.

How would the following observations influence the percent by weight of water in the original salt mixture? Will your value be too large, too small, or not affected? Explain your answer.

a)   Your anhydrous substance was slightly warm when you weighed it.

b) You did not heat long enough and the hydrate was not completely converted to the anhydrous substance.

In an experiment, 2.3754 g of copper(II) sulphate pentahydrate is heated to drive off all the water of crystallisation.

Write a balanced equation.

Determine the mass of anhydrous salt that remains. Show your calculations.

Answer 1