CHEM 231 Quantitative Analysis 1 Spring 2017 Homework 3 You must show working to

Question

CHEM 231 Quantitative Analysis 1 Spring 2017 Homework 3 You must show working to each problems before you can receive a full credit to the question. Correct unit must also be assigned to receive a full credit. Due date: Friday, April 7, 2017 noon in class Name 1. A sulfate fertilizer industry imports its raw material from Mexico. The percentage of sulfate in the raw material must be 2 25 before the raw material can be imported. A 28.589 grams of the raw material was subjected to a gravimetric analysis using barium chloride as a precipitating agent. The weight of the oven dried barium sulfate precipitate at 105 C was found to be 13.55g I. Calculate the weight of sulfate in the precipitate. ll. Calculate the sulfate in the raw material sample. Ill. Based on your result, should the raw material be imported from Mexico?

Explanation / Answer

Ans. #I. 1 mol of barium sulfate, BaSO4 consists of 1 mol SO42-. So, mass of BaSO4 precipitated is always proportional to mass of sulfate ion in sample.

Molar mass of BaSO4 = 233.39 g/ mol

Molar mass of SO42- = (96.06 g/ mol)

Now,

            1 mol BaSO4 ppt. is equivalent to 1 mol SO42-

Or, 233.39 g BaSO4 ppt. is equivalent to 96.06 g SO42-

Or, 1.0 g         -           -           -           -           (96.06 / 233.39) g SO42-

Or, 13.55        -           -           -           -           (96.06 / 233.39) x 13.55 g SO42-

                                                                        = 5.577 g

Therefore, mass of SO42- in dried precipitate = 5.577 g

#II. When treated with BaCl2, the sulfate ion in sample is precipitated as barium sulfate. Assuming complete precipitation as barium sulfate, the amount of sulfate ion in sample is equal to its amount in the precipitate.

Thus, amount of sulfate in sample = amount of sulfate in precipitate = 5.577 g

Now,

% sulfate in raw material = (Mass of sulfate in sample/ Total mass of sample) x 100

                                                = (5.577 g/ 28.589 g) x 100

                                                = 19.51 %

#III. The raw material should NOT be imported because its sulfate content is lower than the desired threshold of minimum 25%.

#IV. % relative error = [ (Actual value – experimental value) / Actual value ] x 100

For per 100 g sample,

            Actual sulfate = 26 % = 26.0 g

            Expected sulfate = 19.51 % = 19.51 g

Or, % relative error = [(26.0 g – 19.51 g) / 26.0 g] x 100

                        = 24.96 %

#V. No. The method of analysis yields very high % relative error.

#Vi. The possible reasons may-

A. Incomplete precipitation of sulfate due to limiting amount of BaCl2 or insufficient time of reaction.

B. Loss of precipitate during its wash.

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