Lab 2 - Lab report: 2 Experiment Title: Gravimetric analysis solution PDF

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Lab report: 2
Experiment Title: Gravimetric analysis solution...

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ABSTRACT

The title of the experiment is Gravimetric Determination of Chloride. The main purpose of the experiment is for us to understand the concept of gravimetric analysis. Besides that, the gravimetric process was done to determine the amount of analyte precipitate produced while predicting or calculating the percentage of the precipitate formed. In the end of the experiment, we would be able to calculate the weight of the analyte by knowing the weight of the precipitate formed from mixing the substances and with our knowledge of its chemical composition. To carry out the experiment, we add the silver nitrate solution into a preheated sodium chloride solution slowly with continuous stirring using a magnetic stirrer. The precipitation process is said to be done when the supernatant liquid starts to clear. After an hour, the precipitate was washed and filtered before drying them on porcelain crucibles in the oven for 1 day. Lastly, the precipitate obtained was weighed. The result obtained from the experiment was …g of silver chloride which is a white precipitate. As a conclusion, the sodium chloride solution reacts with silver nitrate solution to form white precipitate which is silver chloride. Also, from the knowledge of their chemical composition, we know that 1 mol of sodium chloride would react with 1 mol of silver nitrate. After the whole experiment, we notice that a few steps can be done to improve the accuracy of the experiment. The silver nitrate were covered with aluminium foil to prevent oxidation from taking place. Constant stirring is needed during the reaction to prevent bumping of the solution. The precipitate formed has to be washed with concentrated nitric acid and properly filtered. Lastly, we should let the precipitate to cool down after heating in the oven before weighing because the temperature would affect the weight of the precipitate.

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METHODOLOGY 1. Precipitation Chart 1 About 0.2-0.3g of sample solid sodium chloride was weighed into 400ml beaker.

An excess of 10% of silver nitrate solution is added slowly until the coagulation process stop.

The sample is dissolved in distilled water and diluted to about 150ml. About 0.5ml of concentrated nitric acid is added.

The sodium chloride solution is heated to boil with constant stirring.

The precipitate is let to settle. Completion of precipitation is tested by carefully adding a few drops of silver nitrate to clear the supernatant liquid.

The amoun silver nitrate required to make 150ml of 0.1M silver nitrate solution is calculated.

The beaker with their contents were covered with aluminium foil and left standing on the desk for 1 hour.

2. Filtration and washing of the precipitate

Chart The solution was decanted and poured down a stirring rod.

The precipitate was brought on the filter paper onto a porcelain crucible.

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About 25ml of the wash solution is added to the precipitate left in the beaker and stirred well.

The solution is decanted through a filter paper after the precipitate is settled.

Chart 3 The crucible containing the precipitate is placed in the oven for 1 day at 40-50 0C.

The crucible is cooled and weighed

RESULT The result obtained was tabulated in the table as shown below. TABLE 1

Content Mass of sample, NaCl

Mass (g) 0.254

Mass of filter paper

1.052

Mass of porcelain crucible

40.036

Mass of precipitate AgCl + filter paper + porcelain crucible Mass of precipitate AgCl

41.686 0.602

DISCUSSION

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1. Explain the function of wash solution (HNO3) during washing step. The wash solution (HNO3) maintainsa high electrolyte concentration and the AgCl particles stay clotted together to avoid redissolvation of the precipitate. 2. Calculate the percentage of chloride in the sample. −¿ → AgCl(s) ¿ +¿+Cl(aq) ¿ Ag(aq) Number of moles of Cl- : One mol of chloride ion is required to form one mol of silver chloride. Number of moles of AgCl=

¿

Mass of AgCl Molar mass of AgCl

0.602 g 143.4 g /mol

¿ 0.0042 mol

Mass of Cl- in sample: Mass of Cl- = Number of moles of Cl- × Molar mass of Cl¿ 0.0042 mol ×35.5 g/mol ¿ 0.1491 g Percentage of Cl- in sample, %: Mass of

¿

Cl−¿∈sample(g ) ×100 % Mass of sample(g) −¿∈sample , %=¿ ¿ Percentage of Cl

0.1491 g × 100 % 0.254 g

¿ 58.7 %

3. How to obtain large precipitate during the experiment? Heat the solution until it is about to boil, then stir the solution vigorously for about 10 minutes. This step is necessary in order to obtain the large precipitate. The solution also has to be treated with precipitating reagent to the correct pH value to ensure the correct precipitate is formed and also to suppress the formation of other precipitates. 4

CONCLUSION AND RECOMMENDATIONS

Based on the above results, we are able to conclude that the mass of precipitate formed is 0.602g and the mass of Cl - is 0.1491g. The percentage of chloride in the sample, sodium chloride is 58.7%. To ensure that the precipitates formed are large and easily filtered, the precipitate was heated until it almost boils and digestion is allowed to take place for a period of times. So, there are recommendations we have obtained from this experiment. First of all, do not expose the silver nitrate solution to light to prevent light reaction which may give the wrong results. Next, the precipitate is heated and digestion is allowed to take place with constant stirring to ensure the reactants have completely reacted to for the precipitate. After that, the precipitate must be washed and filtered before it is heated. After the heating process is complete, it should be left to cool for about several minutes before the weighing process takes place.

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REFERENCES



Pearson Longman PRE-U PHYSICAL CHEMISTRY TEXT Book.



Chang, Raymond (2007). Chemistry 3rd Edition. New York: McGraw-Hill



Experimental Organic Chemistry A Small-Scale Approach by Charles F.Wilcox,Jr. and Mary F.Wilcox 2nd Edition.



Introduction to Organic Laboratory Techniques: A Small-Scale Approach by Donald L.Pavia – Science 2004.

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APPENDICES

Chart 1 : Procedure of Precipitation Process Chart 2 : Procedure of Filtration and Precipitate Washing Process Chart 3 : Procedure of Drying and Weighing of the Precipitate.

Table 1 : Mass of Various Contents

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