Gravimetric - experiment PDF

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ABSTRACT

This experiment is about gravimetric analysis of solution. There are three objectives in this experiment and there are to understand the concept of gravimetric analysis, to determine the amount of analyte precipitate formed and also to predict the percentage of analyte precipitate formed. First and foremost, all the materials and apparatus are prepared well before the experiment start. The gravimetric analysis involved a series of methods and there are precipitation, filtration and washing of the precipitate formed and finally drying and weighing the precipitate formed. During the precipitation process, 0.26g of sodium chloride had been weighed and put into a 400ml beaker. The sample is dissolved and diluted to 150ml with distilled water. 0.5ml of concentrated nitric acid is then added to the solution. Then, 2.55g of silver nitrate is weighed and dissolved in distilled water. After that, this solution required to put into a burette. The sample that contains sodium chloride and nitric acid is then boiled. During heating of the sample, constant stirring is a important step to prevent bumping of the solution and loss of precipitation while silver nitrate is added into the sample. The precipitate is allowed to settle down. The beaker which containing the solution is then cover with aluminium foil and let it cooled down for 1 hour. The precipitate is then wash and pour through a filter paper. After the filtration is done, the precipitate collected is transfer into another filter paper and is then put into a porcelain crucible. The crucible containing the precipitate is then put into the oven for 1 day. The weight of the crucible is measured after the sample dry and cooled. Based on the result, we knew that the gravimetric analysis is possible to get the amount of analyte precipitate and the percentage of analyte such as chloride in the sample which is pure compound.

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INTRODUCTION

Gravimetric analysis is a technique through which the amount of an analyte (the ion being analyzed) can be quantitative determined through the measurement of mass. The principle behind gravimetric analysis is that the mass of an ion in a pure compound can be determined and then used to find the mass percent of the same ion in a known quantity of an impure compound. The steps commonly followed in gravimetric analysis are preparation of a solution containing a known weight of the sample, separation of the desired constituent, weighing the isolated constituent, and computation of the amount of the particular constituent in the sample from the observed weight of the isolated substance. There

are

four

main

types

of

this

method

analysis

which

are precipitation, volatilization, electro-analytical and miscellaneous physical method. Precipitation gravimetry is used in this experiment as it separates ions from a solution. The

chemical

that

is

added

to

cause

the

precipitation

is

called

the precipitant or precipitating agent. The solid precipitate can be separated from the liquid components using filtration and the mass of the solid can be used along with the balanced chemical equation to calculate the amount or concentration of ionic compounds in solution. An example of a gravimetric analysis is the determination of chloride in a compound. In order to do a gravimetric analysis, a cation must be found that forms an insoluble compound with chloride. This compound must also be pure and easily filtered. The solubility rules indicate that Ag +, Pb 2+, and Hg 22+ form insoluble chlorides. Therefore silver chloride could be used to determine % Cl-, because it is insoluble (that is, about 99.9% of the silver is converted to AgCl) and it can be formed pure and is easily filtered. By using gravimetric analysis, it provides exceedingly precise analysis. It also can be used to determine the atomic masses of many elements to six-figure accuracy. It provides a little room for instrumental error and does not require a series of standards for calculation of an unknown. However, It usually provides only for the analysis of a single element, or a limited group of elements, at a time.

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OBJECTIVE 1. To understand the concept of gravimetric analysis. 2. To determine amount of analyte precipitate. 3. To predict the percentage of analyte precipitate.

METHODOLOGY Apparatus : burette , beaker , stirring rod , filter paper , porcelain crucibles , watch glass , magnetic stirrer , hot plate , weighing bottle Chemicals : concentrated nitric acid ( HNO 3 ) , silver nitrate (AgNO 3 )(solid) , sodium chloride (NaCl )(solid) A. Precipitation

0.2-0.3 g solid NaCl was dissolved in distilled water and 0.5 mL of HN03 was added

The sample was heated with constant stirring

Mass of AgNO₃ powder needed to make 150 mL of 0.1M AgNO₃ solution is calculated and dissolved in beaker

The beaker is covered with aluminium foil and let to cool off for 1 hour

The precipitate is let to be settled and a few drops of silver nitrate solution is added

AgNO₃ was added slowly with an excess of 10% until coagulation process stopped

B. Filtration and washing of the precipitate

The solution is poured down a stirring rod into beaker through filter funnel

25 mL of wash solution was added into the beaker

The solution was decant through the filter paper

The percipitate is brought onto the filter and was put into porcelain crucibles

C. Drying and weighing of the precipitate The crucibles was placed in a covered beaker in the oven for 1 day at 40-50℃

The crucible was cooled and weighed

RESULT TABULATION OF DATA Sample Mass(g) Mass of NaCl sample 0.26 Mass of silver nitrate, AgNo3 powder 2.55 Mass of one filter paper 1.203 Mass of AgCl Precipitate 1.864 Mass of Cl in Sample 0.661 DISCUSSION 1.) Calculate the percentage of chloride in the sample. The percent of chloride in the sample is calculated as follow: mass of Cl ∈sample x 100% % Cl = mass of sample The mass of Cl can be determined from the fact that it is precipitated and weighed as insoluble AgCl. The % composition of Cl in AgCl calculated as follow:

Mass of Cl in sample = mass of AgCl x

Atomic weig h t of Cl Molecular weig h t of AgCl 35.453 107.870 + 35.453

= (1.864 – 1.203) x = 0.1635g Thus ,by substitution: % Cl =

mass of AgCl∈sample mass of sample

x

Atomic weig h t of Cl Molecular weig h t of AgCl

100% =

(1.864−1.203) x 0.26

35.453 x 100% 107.87 + 35.453

= 62.887% The percentage of chlorine by theory is calculated as follow: Atomic weigh t of Cl % Cl = x 100% Mo lecular weig h t of AgCl =

35.453 22.990 + 35.453

x 100%

X

= 60.66%

Gravimetric analysis is used to determine the amount of a substance by finding its mass, and then using the mass to find the quantity of the substance. One of the most common techniques of applying gravimetric analysis is obtaining a precipitate from a solution and removing any impurities from it, in order to find its net mass. In an ideal world, an analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered. The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed. 2.) Discuss the potential of experimental errors. There are some possible error that could happen and affects the results of experiment, which are some precipitate may have been left in the beaker or flask during transfer. Other than that, the last potential source of error could have been NaCl being released when the gas and water were released during the heating. The gas and water escaped rapidly when the mixture of sodium bicarbonate and hydrochloric acid were heated. If too much NaCl were released, then the percent yield would be less. Incomplete precipitation reaction also might be the possibility of the potential error the could happen. Impurities in precipitate also can lead to the potential error, surface adsorption here unwanted material is adsorbed onto the surface of the precipitate. Digestion of a precipitate reduces the amount of surface area and hence the area available for surface adsorption. Washing can also remove surface material. Post precipitate also might be the factors of error. Sometimes a precipitate standing in contact with the mother liquor becomes contaminated by the precipitation of an impurity on top of the desired precipitate. Lastly, generally the solids are dried at about 120 o C but condition for drying can vary considerably. To determine the correct drying regime, a thermogravimetric balance may be used. 3.

) Explain methods to avoid the potential of experimental errors in question (2). There are some methods to avoid the potential of experimental errors. When there is an incomplete precipitation reaction, use an excess of one reactant or make sure the solution is saturated. The reaction may take time so we are required to be patient and make sure the reaction takes place completely. Worst case is that the reaction does not

actually go to completion. In this case, you would have to filter it and add excess of one reactant that is soluble, so you can increase the amount that will react.

CONCLUSION In this experiment, we understand that the gravimetric analysis is a technique through which the amount of an analyte which is known as ion in a pure compound can be determined by the measurement of mass. Then, gravimetric analysis is a process that including precipitation, washing the precipitate, filtration, drying and weighting the precipitate. Overall, we concluded that the amount of analyte like chloride able to measure by using gravimetric analysis and it is determined by measurement of mass. Then, the percentage of analyte like chloride also can get by using gravimetric analysis. The mass of chloride in the sample is 0.1635g and the percentage of chloride in the sample is 62.89%. To ensure that the precipitate formed are large and can be filter easily, the precipitate is required to heat then cool down for a period of times. In conclusion, gravimetric analysis is a technique that able to determine the amount of chloride in the sample and the percentage of the chloride in the sample. For recommendation, there are things that we should take precaution in this experiment for example always use a glove to prevent silver nitrate in contact with our skin or other parts of body. Not only that, the filter paper must be folded into quarter and fit nicely in the filter funnel when performing filtration. Besides that, we must make sure that the solution filtered in the beaker is in a clear solution and is cooled down to get a more accurate result. Not to mention that we encounter some problems during this experiment as the filter paper was torn when we try to remove it from the filter funnel to crucible. Hence, we are required to be careful to avoid spilling. Lastly, always make sure that the precipitate is dried to constant mass before weighing and recording it in table....