Gravimetric determination of water in hydrated magnesium chloride

Essay by yodasfriend March 2002

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Aim: To calculate the value of n in MgCl2.nH2O

Theory: Gravimetric analysis is where the amount of a specific chemical in a material is determined by converting it to a product which can be isolated completely and weighed. This is relevant to this experiment as the amount of water is an unknown value which is to be found by carrying out the analysis.

Heating to constant mass is when repeated heating, cooling then weighing is carried out until the mass of the substance is constant.

The number of moles of water in a substance can be found by measuring the loss in mass on heating.

MgCl2.nH2O MgCl2.nH2O

'n' can be calculated by comparing the mass of hydrated salt with the mass of anhydrous salt by heating to constant mass.

Apparatus: The equipment required is:

a porcelain crucible and lid


pipe-clay triangle

bunsen burner and heating mat



a balance

hydrated barium chloride.

Method: To get rid of any water the crucible was heated, then allowed to cool in a desiccator and its mass was noted (M1). 2-3g of magnesium chloride was then added and the crucible reweighed (M2). The crucible and its contents were then reheated for 15 minutes, allowed to cool and again weighed (M3). The crucible and its contents were then heated, allowed to cool and weighed again, (M4), to make sure the loss in mass was complete. This step was repeated until M4 and M5 were constant.

Results: M1 = 39.59g

M2 = 41.62g

M3 = 40.64g

M4 = 40.63g

Average mass of crucible and anhydrous MgCl2 = 40.635

Mass of anhydrous MgCl2 (M4 - M1) = 1.045g

Number of moles of MgCl2 (GFM = 95.3g) = 1.045/95.3

= 0.012moles

Mass of H2O removed M2 - M4 = 0.985g

Number of moles...