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


I was required to calculate and answer provided questions


Experiment 3: Empirical Formula of Epsom Salt
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 • 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 • 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 (39.1×2 (for K2)) + (32 (for sulphur ions)) + (16× 4(for oxygen ions)). Work out gives, 78.2 + 32 + 64 =174.2. Similarly, relative molecular mass of H2O molecules is (2 × 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 × 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 ×100=50.82 %
To determine the number of water molecules (water of crystallisation) in a hydrated magnesium sulphate.
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.
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) →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.
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...
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