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# Experiment Assignment: Empirical Formula of Epsom Salt (Lab Report Sample)

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Experiment 3: Empirical Formula of Epsom Salt
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Pre-lab questions
1 What do you understand by the terms (a) hydrated, (b) anhydrous, (c) water of crystallisation?
My Response
A hydrated substance contains the water of crystallization it is empirical formulae. On the contrast, anhydrous substance lacks water of crystallization it is structure. Water of crystallization refers to necessary water molecules that are required for the formation of crystals by some substances.
2 Copper nitrate is a hydrate with the following formula: Cu(NO3) 2 â€¢ 3 H2O. What is the ratio between moles of copper nitrate and moles of water in this hydrate?
My Working
There is one mole of Cu(NO3) 2 and 3 moles of H2O molecules in the formula Cu(NO3) 2 â€¢ 3 H2O. Therefore, the ratio of Cu(NO3) 2:H2O in the hydrated salt is 1:3.
3 What is the percent of water in K2SO4 .10H2O?
My Working
The relative formula mass of anhydrous K2SO4, is (39.1Ã—2 (for K2)) + (32 (for sulphur ions)) + (16Ã— 4(for oxygen ions)). Work out gives, 78.2 + 32 + 64 =174.2. Similarly, relative molecular mass of H2O molecules is (2 Ã— 2 (for H2)) + 16 (for an oxygen ion) giving a total value of 18. However, there are 10 water molecules in hydrated potassium sulphate, thus total mass of water molecules is 18 Ã— 10 = 180. Therefore, total formula mass of K2SO4 .10H2O is 174.2+180=354.2 atomic units. Percentage composition of water in K2SO4 .10H2O is:
180354.2 Ã—100=50.82 %
Objective
To determine the number of water molecules (water of crystallisation) in a hydrated magnesium sulphate.
Introduction
Crystalline solids tend to have stoichiometric quantity of water of crystallisation in their structure. Salts that contains water of crystallisation are referred hydrated salts; whereas, those deficient of water of crystallisation are referred to as anhydrous salts. An example of an hydrated salt is the hydrated magnesium sulphate (Epsom salt). The experiment involved determination of the number of water of crystallisation in hydrated Epsom salt i.e. the value of x in the molecular formula, MgSO4.xH2O.
Principle
Heating of a hydrated salt drives out water of crystallisation in a hydrated leaving behind anhydrous salt. A change in weight is equivalent to the weight of water of crystallisation in the hydrated salt sample. This allows for determination of the empirical formula mass. Similarly, when Epsom salts are heated, their water of crystallisation is driven off, leaving anhydrous magnesium sulphate, MgSO4
MgSO4.xH2O(s) â†’MgSO4(s) + xH2O(g)
Or generallyÂ
Hydrated saltÂ  Â Â anhydrous saltÂ +Â water vapour
Since many hydrates contain water in a stoichiometric quantity, it is possible to determine the molar ratio of water to salt.
Materials and Methods
The main aim in the experiment was to determine the empirical formula of the hydrated magnesium sulphate (MgSO4.xH2O). Firstly, a dry and clean boiling test tube with cork stopper was weighed and its mass recorded. It was followed, by addition of hydrated magnesium sulphate into the boiling test tube to a depth of approximately 1cm, the stopper was put into place and reweighed and mass value recorded. The stopper was then removed as the test tube, containing hydrated magnesium sulphate, was clamped on a retort stand and subsequently heated. The heating process lasted for a period of 2 minutes on gently heating, and 8 minutes for strong heating, after which, the Bunsen burner was turned off and the boiling test tube cork stoppered. The tube was allowed to cool then reweighed again, results recoded. The tube was then reheated again, in the same version, for 5 minutes, and its mass weighed again.
Results and Calculations
Weight of test tube and stopper 45.5107g
Weight of stopper test tube and sample before heating 51.4911g
Weight of sample 5.9804g
Weight of stopper test tube and sample after first heating48.4300g
Weight of stopper test tube and sample after heating to constant weight 48.4259g
Weight difference of first and second heating0.0041g
The mass of used hydrated MgSO4.xH2O was 5.9804g, while the mass of the anhydrous MgSO4 was (48.4259g â€“ 45.5107g = 2.9152g) 2.9152g. Therefore, mass of water of crystallisation in the sample was determined to be (5.9804g â€“ 2.9152g =3.0652g) 3.0652g.
Percentage by mass of water of crystallisation in hydrated MgSO4.xH2O was: 3.0652g5.9804Ã—100=51.25%
The number of moles of water in the hydrated MgSO4.xH2O sample was: 3.0652g18g/mol=0.1703 moles
Formula mass of anhydrous MgSO4 was Mg + S + O Ã—4, i.e. 24 + 32 + 16 Ã—4 = 120. Thereby, mole of anhydrous salt was: 2.9152g120g/mole=0.0243 moles. The mole ratio of MgSO4:H2O was determined as follows: 0.0243/0.0243: 0.1703/0.0243, giving as refined ratio of 1:7. Therefore the relative molecular mass of hydrated magnesium sulphate is given as MgSO4.7H2O.
Experimental Observations
On gently heating, a clear colourless liquid appeared to form on the upper sides of the test boiling test tube. On strong heating the colourless liquid completely evaporated leaving the tube dry and a white residue at the base of the boiling test tube. It was evident that the former crystalline and transparent solids of the hydrated magnesium sulphate turned into a white powder after heating.
Discussion
The colourless liquid released from heated hydrated magnesium sulphate was water of crystallisation. The change in appearance by the hydrated magnesium, on heating, from a crystalline clear symmetrical salt into an amorphous white powder was due to transformation of the previously hydrated salt into an anhydrous product. Some of the identifiable errors in the experiment included errors due to mass measurement, possible presence of moisture in the heated sample, error due to time measurement and possible impurity of the hydrated magnesium sulphate salt. The objective of the experiment was met as the empirical formula of the Epsom salt was found to be MgSO4.7H2O.
Post Laboratory Questions
1 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.
My Response
I would utilise the anhydrous cobalt (II) chloride. If the liquid being given off is water then anhydrous cobalt (II) chloride paper will turn from blue to pink.
2 Why might you need to reheat the salt several times before getting the final mass of the salt?
My Response
The hydrated salt sample should be reheated to ensure that all water of crystallisation had be driven out, to facilitate determination of accurate results.
3 What is the percent composition of oxygen in Al2(SO4)3? Show working.
My Response
The formula mass of Al2(SO4)3 i.e 2 Ã—Al + 3(S + 4 Ã—O) is given as 2 Ã—27 + 3(32+ 4 Ã—16) = 342. However, mass of O in Al2(SO4)3 is 192 atomic units. Consequently, percentage composition of O will be 192342 Ã—100=56.14%.
4 Analysis of a sample of hydrated salt shows that it contains 4.00g Ca, 7.09g Cl, and 3.60g H2O.
* What is the formula of the hydrated salt? Show working.
My Working
Element/Molecule

Ca

Cl

H2O

Composi...
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