Lab Report

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Name: Tan Zuen Rian
Admin No.: 1155592
Class: 1A/02
Experiment 4:
Quantitative Determination of Sulphate by Gravimetric Analysis

This experiment aims to determine the amount of SO42- via the gravimetric method. By this way, we will be able to familiarize ourselves with the procedures of gravimetric analysis. Precipitation of BaSO4 is done by acidifying the diluted sulphate solution and then adding BaCl2 solution after heating it for some time. A test for complete precipitation is done by adding a few drops of BaCl2 solution after digesting the solution for 20 minutes. The clear supernatant liquid is filtered by gravity filtration using a vacuum pump. It is dried in an oven and a desiccator and finally weighed. The result obtained is 0.2005g, which is more than expected, thus proving the presence of impurities in the precipitate. Thus, measures have to be taken to ensure a lower concentration of impurities. Objective:

The objective of this experiment is to determine the amount of sulphate in a sulphate solution by using the gravimetric method.

The amount of sulphate is determined quantitatively as barium sulphate, BaSO4, by gravimetric analysis. This determination consists of slowly adding a dilute solution of barium chloride to a hot (250oC) unknown sulphate solution slightly acidified with concentrated hydrochloric acid (12M). The method used here is gravimetric analysis. It is a method based on isolating a substance required, either in pure form or in a combined form from a sample. Once the substance is isolated successfully, it is then weighed. It is known that solubility differs for all compounds. As such, there are compounds that are practically insoluble. As such, precipitation is used. Two of the common inorganic precipitating agents are silver nitrate (AgNO3), used to precipitate halide ions such as chloride ions. Another one would be barium chloride (BaCl2), used to precipitate sulphate ions, as shown in this experiment. Ba2+ + SO42- BaSO4 (ppt.)

The white precipitate is filtered, washed with water, dried in the oven (150oC) and weighed as barium sulphate. Advantages:
Gravimetric analysis gives a very precise analysis if the methods are followed strictly. Moreover, it was used to determine the relative atomic masses of many elements to a high degree of accuracy. At the same time, the probability for errors is very low. Also, the equipment required in gravimetric analysis is in expensive.

However, this method only analyzes a single element, or a limited number of elements. As such there are constantly new equipment and technology that allows multiple analyses. The traditional methods have to be followed strictly as one wrong step would give undesirable results, thus giving rise to a need to restart the experiment.

(A) Precipitation of BaSO4
1. 25ml of the given sulphate solution was pipetted and placed into a 250-ml beaker. 2. 50ml of water, measured in a measuring cylinder, was added, after which, concentrated HCl was added in the fume hood. 3. The beaker was placed on a heating pan and heated to 250oC and 10ml of 10% barium chloride solution was added drop wise from a measuring cylinder. Vigorous stirring was done at the same time. 4. A watch glass was placed on the beaker to cover it and it was further heated for 20 minutes at the same temperature. 5. A few drops of barium chloride solution were added to the clear supernatant liquid to test for complete precipitation.

(B) Washing and filtration of BaSO4 precipitate
1. Small pieces of filter paper were placed into the crucible and small amounts of warm water were added to ensure that the filter paper is attached to the base of the crucible. 2. The crucible was attached to a special conical flask (airtight) with an extra hole to be plugged into the vacuum pump. 3. The clear supernatant liquid was slowly poured into the crucible while the conical flask was...
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