Experiment: Gravimetric Analysis 9j42

TITLE: The Gravimetric Determination of Chloride

OBJECTIVES OF EXPERIMENT: After completing the experiment, the students should be able to: 1. List down the important factors which effect the crystallinity/size of precipitate. 2. Describe the steps to be taken to ensure that the precipitates formed are large and easily

filtered and avoid peptization. 3. Carry out the gravimetric experiment confidently. 4. Calculate the theoretical yield of a product. 5. Calculate the percentage yield of a product.

INTRODUCTION:

Gravimetric methods are among the most accurate and precise methods of quantitative analysis. These advantages are counterbalanced by the fact that they are often time consuming, require considerable attention to detail and are limited to sample size and concentrations which yield a weighable quantity on a conventional analytical balance.

While many gravimetric

methods have been at least partially replaced by newer, faster techniques, there remain several situations where the only suitable analytical techniques are gravimetric. Addition of a solution containing silver ion [usually an aqueous silver nitrate (AgNO3) solution] to a dissolved sample containing chloride ion will quantitatively precipitate the chloride ion as solid silver chloride (AgCl), due to the small solubility product constant of silver chloride (ksp = 1.82 x 10-10). The precipitate may be isolated from solution by filtration, the material dried, weighed and the percent chloride (% Cl-) calculated from the stoichiometry of the reaction:

+

Ag (aq) + Cl- (aq)

AgCl(s)

In addition to possible interferences common to all gravimetric methods, the presence of any other halide will cause high results since they produce a silver halide which is even more insoluble than silver chloride.

Also, silver chloride is light sensitive and excessive

photodecomposition will produce erroneous results, according to the reaction:

AgCl(s)

hv

Ag(s) +

1

2

Cl2 (g)

The precipitate becomes violet-purple, due to the presence of finely divided silver metal, and results will be low. If silver ion (Ag+) is present, in addition to the above, the following reaction will also occur as a result of the photodecomposition reaction:

hv 3Cl 2 (g) + 3H 2 O(l) + 5Ag + (aq) → 5AgCl(s)

If this reaction predominates, the results will be high.

+ ClO3- (aq) + 6H + (aq) To minimize such errors, it is

recommended that unnecessary exposure to light be avoided. Gravimetric analysis, if methods are followed carefully, provides for exceedingly precise analysis. In fact, gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy. Gravimetry provides very little room for instrumental error and does not require a series of standards for calculation of an unknown. Methods also do not require often expensive equipment. Gravimetric analysis, due to its high degree of accuracy, when performed correctly, can also be used to calibrate other instruments in lieu of reference standards.

METHODOLOGY:

RESULT: Weight of dried NaCl: Sample 1: 0.3029 g Sample 2: 0.3001 g Sample

1

2

Initial reading (cm3)

0.30

1.80

Final reading (cm3)

17.60

17.30

Volume of AgNO3 used(cm3)

17.30

15.50

1

2

74.0715

46.8966

74.5413

47.3095

0.4698

0.4129

Sample Weight of crucible + filter paper (g) Weight of crucible + filter paper + precipitate (g) Weight of precipitate (g)

CALCULATION: A. Theoretical Data 1. Minimum volume of AgNO3

Mr of NaCl = (22.989770 + 35.453) g/mol = 58.44277 g/mol Mr of AgCl = (107.8682 + 35.453) g/mol = 143.3212 g/mol Mr of Cl- = 35.453 g/mol

Na+ (aq) + Cl- (aq) → NaCl(aq)

2. Equation:

NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq) 3. n NaCl = Mass NaClMr of NaCl = 0.3 g58.44277 g/mol = 0.00513 mol 4. From equation, 1 mol of Cl- ≡ 1 mol of NaCl

Mass of Cl- = n Cl- × Mr Cl= 0.00513 mol × 35.453 g/mol = 0.1819 g 5.

Theoretical percentage of Cl- in NaCl = Mass Cl-Mass of NaCl × 100 % = 0.1819 g0.30 g × 100 % = 60.63 %

6.

Minimum volume of 0.2 M of AgNO3 required: 1 mol of NaCl ≡ 1 mol of AgNO3 n NaCl = 0.00513 mol n = MV1000 V = 1000 nM = 1000 (0.00513 mol)0.2 M = 26.65 cm3

B. Sample 1 1. n AgCl = Mass AgClMr of AgCl = 0.4698 g143.3212 g/mol = 0.00328 mol 2. NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)

Ag+ (aq) + Cl- (aq) → AgCl (s) 1 mol AgCl ≡ 1 mol ClMass of Cl- = n Cl- × Mr Cl= 0.00328 mol × 35.453 g/mol = 0.1163 g 3.

Percentage of Cl- obtained = Mass Cl-Mass of NaCl 100 % = 0.1163 g0.3029 g ×

100 % = 38.40 % 4.

Percentage of error = | Theoretical % Cl- -Experimental % of Cl-Theoretical %

Cl- | × 100%

= | 60.63 -38.4060.63 | × 100% = 36.67 % Sample 2 1.

n AgCl = Mass AgClMr of AgCl = 0.4129 g143.3212 g/mol = 0.00288 mol

2.

NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq) Ag+ (aq) + Cl- (aq) → AgCl (s) 1 mol AgCl ≡ 1 mol ClMass of Cl- = n Cl- × Mr Cl= 0.00288 mol × 35.453 g/mol = 0.1021 g

3.

Percentage of Cl- obtained = Mass Cl-Mass of NaCl 100 % = 0.1021 g0.3001 g ×

100 % = 34.02 % 4.

Percentage of error = | Theoretical % Cl- -Experimental % of Cl-Theoretical %

Cl- | × 100%

= | 60.63 -34.0260.63 | × 100% = 43.89 %

DISCUSSION: 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. Stoichiometry is the study of the relationships between the quantities of reactants and products involved in chemical reactions, and is used in many calculations involving molar and mass ratios. Gravimetric stoichiometry is a combination of gravimetric analysis and stoichiometry, and in short it is the procedure for calculating the masses of reactants, or products, in a chemical reaction. Gravimetric stoichiometry is used in this experiment to calculate the theoretical yield of a chemical reaction,

and the percentage yield (what percentage of the amount predicted for a substance was actually produced). The objective of this experiment is to obtain percentage yield of chloride ions in a solution of sodium chloride, using silver nitrate as a reagent. When sodium chloride is added to silver nitrate, a double displacement reaction occur which results in silver chloride and sodium nitrate. The experiment was started by dissolving dried NaCl salt in distilled water and 6 M HNO3 solution. In this solvation process, acid solution has been added first in distilled water before adding the salt so that the salt completely dissolving in homogenous dilute acid solution. The addition of AgNO3 solution to form precipitate of AgCl was carried out slowly and with continuous stirring to let the reaction completion. If not enough time for the reaction to complete, a lower yield of the precipitate would occur. More actions were taken so that ‘good’ precipitate formed; which heating the precipitates solution and let it to digest then carried out the test for completeness of the precipitation. As the reaction may involve photodecomposition of AgCl salt, the salt was left to continue its reaction under darkness by covering its container with aluminium paper and put into laboratory table which no light could enter. During the filtration process, solution of HNO3 was used to wash the precipitates thoroughly to decrease the possibility of peptisation of the salts. The washing process also needed rubber policemen to remove any particles that adhere to the walls of the container. The washing steps was free of Ag+ ions when no more cloudy solution seen in the waste solution of the wash. After the calculation, the percentage yield of AgCl precipitate was 38.40 % in Sample 1 while in Sample 2 was 34.02 % from the theoretical value. The high percentage of errors, for Sample 1 was 36.67 % and 43.89 % for Sample 2 mainly induced by one step missed which was

incomplete precipitation reaction due to not adding enough precipitating agent(AgNO3). Minimum amount of the precipitating agent should be used was 26.65 cm3. However in this experiment, the minimum amount was not reached resulting in low percentage yield of products. Therefore, the experiment can be improved by avoiding these gravimetric errors that leading to a lower mass of precipitates. One more thing that was important, always weight the dried precipitates so the correct value was recorded.

PRECAUTION STEPS: 1. Use AgNO3 carefully and sparingly. It leaves dark stains (metallic silver) on equipment,

skin and laboratory surfaces. If spill occurs wipe up immediately and rinse thoroughly with plenty of water. 2. Crucible must be dried in the oven first to remove moist in its content before using it to reduce reading errors when weighing the precipitates. 3. The precipitates must be dried completely before weighing it to avoid obtaining wrong reading of the precipitates.

CONCLUSION: 1. The crystallinity/size of precipitate is affected by choosing correct reagents with correct

cations. Ag+ ions is most suitable cation to produced large size AgCl precipitates when it coagulates with Cl- ions.

2. To ensure that the precipitates formed are large and easily filtered and avoid peptisation,

these steps are taken: –

The precipitates was heated and let it digest in enough time

–

Washed with volatile reagent such as HNO3 solution to avoid redissolvation of precipitates

1. The experiment was done confidently by the way of gravimetric analysis. 2. The theoretical yield of Cl- calculated is 60.63 %. 3. The percentage yield of Cl- obtained in Sample 1 was 38.40 % while in Sample 2 was

34.02 %. 4. The percentage of errors in experiment for Sample 1 was 36.67 % and 43.89 % for Sample 2. REFERENCES: 1. Gary D.Christian: Analytical Chemistry Sixth Edition, by John Wiley and Sons Inc. 2. Nelson, J., Chemistry:The Central Science, 3rd Edition, Prentice- Hall, 1985.

3. https://vinstan.wikispaces.com/Final+Project+ +Gravimetric+Estimation+of+Chloride+Ions 4. D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch Analytical Chemistry: An Introduction, 7th ed. Chapter 8, pp. 179-201.

QUESTIONS: Explain why in gravimetric analysis of chloride ion using ion silver precipitation: •

An excess of silver ions is added so that the chloride concentration at equilibrium will be negligible. If enough silver nitrate solution is not used, the precipitation will be incomplete, resulting in a substantial error and a low value for the % Cl in the sample.

•

Nitric acid and the small excess of silver nitrate promote coagulation by providing a moderately high electrolyte concentration. Nitric acid in the wash solution maintains the electrolyte concentration and eliminates the possibility of peptization during the wash step; the acid subsequently vaporizes as the precipitate is heated to dryness. Consult your text for additional information concerning the properties and treatment of colloidal precipitates.