Gravimetric Analysis

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  • Topic: Gravimetric analysis, Mole, Barium chloride
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SINGAPORE POLYTECHNIC

SCHOOL OF CHEMICAL & LIFE SCIENCES

CP 4001: ANALYTICAL & PHYSICAL CHEMISTRY

Experiment 4: Gravimetric Analysis

Prepared for:

Mr Goh Tong Hng

Submitted by:

Ng Hui Shan (0900931)

DBS/FT/1A/02

26th May 2009

CONTENTS

1. Synopsis 3

2. Objectives 4

3. Theory

1. Experimental Procedure 4

2. Stoichiometric Calculation 7

4. Procedure 7

5. Results & calculations

1. Amount of Sulphate 9

2. Percentage Yield 9

6. Discussions and recommendations

1. Accuracy of Results 11

2. Factors Affecting the Concentration of Sulphate 11

3. Measures to Improve Results 12

7. Conclusion 13

8. References 14

1. SYNOPSIS

The main objective of this experiment was to determine the amount of sulphate by a process called gravimetric analysis. Gravimetric analysis is the quantitative isolation of a substance by precipitation and the weighing of the precipitate. In this experiment, sodium sulphate was allowed to react with barium sulphate to form a white precipitate of barium sulphate. This precipitate was then washed, filtered and dried, to ensure that there are no impurities and water molecules present, before it was being weighed. The amount of sulphate obtained was calculated to be 0.2028g and the percentage yield was calculated to be 98.8%. Due to a number of experimental errors, this value is inaccurate. The percentage yield should be less than that because of the loss of sulphate ions during the experimental procedures. However, with the presence of impurities, the weight of sulphate obtained is greater. Therefore, measures should be taken to reduce the loss of sulphate ions and accumulation of impurities.

2. OBJECTIVES

The main objective of this experiment is to find out the amount of sulphate by using the gravimetric method. The other objectives are to learn how to use a vacuum pump, calculate the weight percent of sulphate in the salt as compared to the theoretical weight.

3. THEORY

3.1. Experimental Procedure

The amount of barium sulphate precipitate was used to determine the amount of sulphate by gravimetric analysis. Gravimetric analysis is the quantitative isolation of a substance by precipitation and the weighing of the precipitate. [1]

The barium chloride solution was used as a source of barium cations, Ba2+. The sodium sulphate solution was used to provide a supply of sulphate anions, SO42-. By adding these two solutions together, an insoluble white precipitate of barium sulphate, BaSO4, will be formed. The ionic equation of this reaction is given by:

Ba2+ + SO42- ( BaSO4 (

Concentrated sulphuric acid was not added to the sodium sulphate solution to slightly acidify it as it will add an excess of sulphate ions to the solution, causing the amount of barium sulphate precipitate obtained at the end of the experiment to be more than the theoretical mass. Therefore, concentrated hydrochloric acid, HCl, was used. HCl prevents the precipitation of barium carbonate, BaCO3 and barium hydroxide, Ba(OH)2.[2] HCl also aids the formation of larger precipitate crystals because fewer crystallite nuclei are formed.[2]

The mixture, containing sodium sulphate solution, HCl and water, was heated to boiling before barium chloride solution was added because at a higher temperature, the molecules will move faster as they slide across each other. When these molecules move faster, they will tend to collide with each other more often. Therefore, when the barium chloride is added into the beaker, more effective collisions will occur, speeding up the formation of BaSO4 precipitate. The barium chloride solution was added drop wise into the beaker because this will allow more time for the SO42- anions to react with the Ba2+ cations to form BaSO4 precipitate. The vigorous stirring of the solution in the beaker will help...
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