Gravimetric An ore sample, 2.9507 g was analyzed for its sulphate content. The s

Question

Gravimetric

An ore sample, 2.9507 g was analyzed for its sulphate content. The sample was dissolved and treated to convert all sculpture to sulphate and then the solution was diluted to 200 mL and heated to boiling. The sulphate in the sample was precipitated with addition of hot Y chloride The precipitate was filtered, washed and dried to remove moisture. The dry weight of X sulphate obtained was 1.2120 g. Write the chemical formula of Y chloride and X sulphate in the method. Find the gravimetric factor for the above determination. Determine the percentage (w/w) of sulphate in the ore sample. In gravimetric method of analysis, the precipitating agent can be added by direct addition or by homogenous production. Compare and contrast two benefits of homogenous precipitation of analytic in sample, to direct addition of the precipitating agent.

Explanation / Answer

1. (i) Chemical formula for Y chloride is BaCl2.

X sulphate = BaSO4

BaCl2 is generally used for precipitation of sulphate and determine S content in sample. When Barium chloride is added to sulphate compound, it precipitates as BaSO4.

(ii) Gravimetric factor = mass of analyte/mass of precipitate

= mass of ore taken/mass of dry sulphate ppt

= 2.9507/1.2120 = 2.435

gravimetric factor = 2.435

(iii) mass of ppt obtained = 1.2120 g

Molar mass of BaSO4 = 233.43 g/mol

moles of BaSO4 = mass/molar mass = 1.2120/233.43 = 0.0052

moles of SO42- = moles of BaSO4 = 0.0052 mol

mass of 1 mol of SO42- = 96 g/mol

mass of SO42- obtained = moles *mass = 0.0052*96 = 0.50 g

% of SO42- = (mass of SO42-/mass of ore taken)*100 = (0.50/2.9507)*100 = 16.95 %

2. Homogeneous precipitation is better than direct addition because in homogeneous precipitation, the precipitating agent is added slowly and there is proper mixing of analyte and agent to produce precipitates. The advantages are more precipitates are formed and purity of precipitates formed. If direct addition is done, then proper mixing does not occur and reaction is not 100 % complete. So, less precipitates are formed leading to wrong results and then precipitates formed will also be not pure.

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