The Formula of Epsom Salt Lab Report

Anthony Bui

2014-09-17

The formula of Epsom salt Lab Partner: Christoffer Lundgren Lab Instructor: Henrik Engström Date performed: September 10, 2014 Date submitted: September 17, 2014 Aim: Epsom salts are sold as health salts. They are hydrated magnesium sulphate. That means they contain water, called water of crystallization, as well as magnesium sulphate. The formula can be written as MgSO4H2O, and the aim is to find the value for x in the formula of Epsom salt. When Epsom salts are heated, the water of crystallization is driven off leaving anhydrous magnesium sulphate, MgSO4. Chemicals and Equipment: Epsom salt, crucible, tripod, triangle, spatula, Bunsen burner, balance Procedure Record all you weightings in a data table 1. Weight a clean and dry crucible 2. Add approximately 0.5 to 1 cm depth of Epsom salt and note the mass. 3. Place the crucible in the triangle on the tripod. 4. Heat gently at first then strongly for about 5 minutes. 5. Let the crucible cool to about 30°C. Weigh. 6. Heat again for a couple of minutes. Allow to cool and weigh. 7. If the last two weightings differ by more than 0.05 g, repeat the heating and weighting. 8. Calculate a value for x in MgSO4H2O. Mass of crucible: 30.24 g Mass of crucible and salt: 37.03 g Mass of salt: 6.79 g Before heating Mass of 6.79 substance (g ±0.01) Mass difference 0.00 in substance (g ±0.01)

After 1st heating 3.87

After 2nd heating 3.10

After 3rd heating 3.33

-2.92

-0.77

+0.23

Anthony Bui

2014-09-17

Qualitative data: - Bubbling - Sound and looks like it’s boiling These two stopped after 2 minutes at medium burn - Center looks like it’s melted Calculations For the calculations we are going to use the measurements from the second heating due to the fact that after the third heating our substance gained some mass even though we were trying to remove as much water mass as possible. Seeing as it gained mass, the third measurement is not as dependable as the second. Molar mass of MgSO4 = 120.367 g Molar mass of H20 = 18.015 Total lost mass after 2nd heating = 3.69 g 3.69/18.015 = 0.2048293089 mol Mass of substance after 2nd heating = 3.10 g 3.10/120.367 = 0.0257545673 mol Molar ratios 0.0257545673/0.0257545673 = 1 0.2048293089/0.0257545673 = 7.95 From these calculations we can estimate that the molar ratios are roughly 1 MgSO4 per 8 H2O molecules. This means that the x = 8 Uncertainties due to scale inaccuracy Minimum Total lost mass after 2nd heating = 3.69-0.01 g 3.68/18.015 = 0.2042742159 mol Mass of substance after 2nd heating = 3.10+0.01 g 3.11/120.367 = 0.0258376465 mol 0.0258376465/0.0258376465 = 1 0.2042742159 /0.0258376465 = 7.91 Maximum Total lost mass after 2nd heating = 3.69+0.01 g 3.70/18.015 = 0.2053844019 mol Mass of substance after 2nd heating = 3.10-0.01 g 3.09/120.367 = 0.025671488 mol 0.025671488/0.025671488 = 1 0.2053844019 /0.025671488 = 8.00

Anthony Bui

2014-09-17

From these calculations we can see that the uncertainty of x is between 7.91-8.00 Discussion The first thing that can be noted is the inaccuracy of the experiment. According to the instructions given, if the mass difference of the substance were to be greater than 0.05g after the heating, then we would required to heat the substance once more to ensure that we can heat off as much water as possible. The inaccuracy can easily be narrowed down by performing the heatings repeatedly, however, due to time restrictions this was only possible to do three times. Other than this, there were also limits of equipment (ex. the scale measuring accuracy, which we account for in minimum and maximum) and perhaps human error. The change in mass between the first and second heating was 0.77g as opposed to the maximum amount allowed being 0.05g. This massive difference could be due to various reasons:   

Method of heating along with equipment Reactions with external particles Human error

Apart from human error and the salt sample reacting with external particles, which we think affected the experiment value-wise only by a small bit, we believe that the method of heating the salt sample played a large role in that 0.77g was heated off after the second heating. Because of that the Bunsen burner and the crucible was stationary, not all of the crucible was heated efficiently. This does not necessarily mean that it is a bad method, however, it may have been better to either with an automatic rotator or manually with our hands heat all sides of the crucible equally. Regarding the human error, besides reading off the measurements incorrectly, there may have been other substances mixed in with the salt due to us not being cautious enough, possibly resulting in a change in the mass or the increase in mass as seen in our 2nd heating to 3rd heating.