Gravimetric Methods of Analysis.pdf

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CAPE Chemistry - Unit 2 Lecture Notes – Gravimetric Methods of Analysis

1 Copyright © 2016 Mark December

Gravimetric Analysis

Gravimetric analysis uses the mass of a sample to determine the amount of one substance that is present

in the sample. The main steps in Gravimetric Analysis are:

Preparation of the solution

Precipitation

Digestion

Filtration

Washing

Drying (in a crucible) or igniting (using a flame)

Weighing

Calculations

Types of Gravimetric Methods

Precipitation Gravimetry

In precipitation gravimetry an insoluble compound forms when a precipitating reagent, or

precipitant, is added to a solution containing the analyte. In order for precipitation to be

successful, the precipitate formed must be:

of low solubility,

of high purity,

of known composition if its mass is to accurately reflect the analyte’s mass,

easy to separate from the reaction mixture, and

free from impurities.

Because precipitation usually occurs in a solution that is rich in dissolved solids, the initial

precipitate is often impure. These impurities must be removed before determining the

precipitate’s mass.

Coprecipitation occurs to some degree in every gravimetric analysis. It cannot be avoided but

can be minimized by careful precipitation and thorough washing.

Surface Adsorption occurs when unwanted material is adsorbed onto the surface of the

precipitate. Washing serves to remove surface material.





Practice Question:

1. A certain barium halide exists as the hydrated salt BaX2.2H2O, where X is the halogen. The

barium content of the salt can be determined by gravimetric methods. A sample of the halide

(0.2650 g) was dissolved in water (200 cm3) and excess sulfuric acid added. The mixture was

then heated and held at boiling for 45 minutes. The precipitate (barium sulphate) was filteredoff, washed and dried. Mass of precipitate obtained = 0.2533 g. Determine the identity of X.

(RMM BaSO4= 233.4)





Questions:

1. A 0.497g sample of a chloride of a group 1 metal is dissolved in water. Excess acidified silver

nitrate solution is added to the solution of the chloride. The resulting precipitate is filtered and

dried to constant mass. The mass of AgCl formed is 0.957g. Deduce which metal is present in

the original chloride. (Molar mass of AgCl = 143.5 g/mol)

2. The treatment of a 0.8g sample of impure KCl with excess AgNO3 solution resulted in the

precipitation of 1.46g AgCl. Calculate the % of KCl in the sample. (Molar mass of AgCl = 143.5

g/mol)

3. Deduce the formula of MgClX from the following information: 0.635g of MgClX reacts with

excess AgNO3. The mass of AgCl formed is 1.914g. (Molar mass of AgCl = 143.5 g/mol)

4. A student is given a 2.94g sample of a mixture containing anhydrous MgCl2 and KNO3. To

determine the % by mass if MgCl2 in the mixture, the student uses excess AgNO3 to precipitatethe chloride ions as AgCl. The mass of the precipitate is 5.48g. Calculate the number of moles

of MgCl2 in the original mixture and the % by mass if MgCl2 in the original mixture. (Molar

mass of AgCl = 143.5 g/mol)

5. When 0.611g of hydrate barium chloride is heated to a constant mass, 0.521g of residue are

formed. Deduce the formula of hydrated BaCl2. [RAM: Ba = 137.3, Cl = 35.5, O = 16, H =1]

6. A student collected the following data in an experiment to determine the number of moles of

water in one (1) mole of MgCl2. nH2O.

Mass of empty container = 22.347g

Initial mass of sample = 25.825g

Mass of sample and container after heating = 23.978g

Use the data above to calculate the number of moles of water lost when the sample was heated

and determine the formula of the hydrated compound.

Volatilization Gravimetry

Another approach to gravimetry is to thermally or chemically decompose the sample andmeasure the resulting change in its mass. For example, the mass of water in a sample can be

calculated by heating the sample and finding the loss in mass from the sample.

Another method in volatilization gravimetry involves the trapping and weighing of a volatile

decomposition product. For example the mass of water in a sample can also be calculated by

heating the sample and collecting the vapour in a solid desiccant. The increase in mass of the

desiccant will be equal to the mass of water in the sample. .





The amount of carbon dioxide in a sample of a carbonate can also be determined by

volatilization. Carbonates are decomposed into carbon dioxide which is absorbed by a solid

desiccant such as Ascarite II which is made up of sodium hydroxide in a non-fibrous silicate.

The sodium hydroxide absorbs the CO2 forming sodium carbonate and water.

2NaOH (aq) + CO2 (g) ⇌ Na2CO3 + H2O

Practice Question:

1. A sample of impure calcium carbonate was decomposed with excess hydrochloric acid. The

liberated carbon dioxide, collected in an absorbent was found to weigh 0.88g. Calculate the %

of CaCO3 in the sample.

2. A 8.4g sample of copper (II) carbonate was decomposed with excess sulphuric acid. The

liberated carbon dioxide, collected in an absorbent was found to weigh 2.2g. Calculate the % of

CuCO3 in the sample.

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Gravimetric Methods of Analysis.pdf