1. Volatilisation. 2. Precipitation. 1. Volatilisation

(1)

1 Lesson 5: Gravimetric Analysis

*Read through the lesson notes. You can write them out, print them or save them.

*Once you have tried to understand the lesson answer the questions that follow and self-evaluate your work by checking the answers.

Learning Intention

-Learn about the two types of gravimetric analysis (volatilisation and precipitation).

-Practice calculations involving gravimetric analysis

Background

From National 5 Chemistry, we have met the concept of precipitation. This normally involves reacting two soluble substances to produce an insoluble substance. The insoluble substance can be filtered off, dried and further analysed.

Gravimetric Analysis

Gravimetric analysis is based on the measurement of mass. It involves the accurate measurement of the mass of a reaction product from an accurately measured mass of a reactant. A substance is converted into another substance of known chemical composition, which can be readily isolated and purified. This is normally carried out by:

1. Volatilisation 2. Precipitation 1. Volatilisation

In volatilisation conversion, a substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded.

A common laboratory experiment of volatilisation is the analysis of a hydrated salt, e.g. hydrated copper sulfate, CuSO

4

.nH

2

O. Many salts are present in the hydrated form, meaning that they have a certain number of water molecules.

If the water is “driven off” then it is said that the water molecules are no

longer present and that the salt is in the anhydrous (without water) form. In

this particular example, an analytical chemist may be asked to work out the

number, n, of water molecules present in the hydrated form of the copper

sulfate salt.

(2)

2 Porcelain crucible being heated using a strong blue flame.

Desiccator Analytical balance

A small porcelain crucible is heated using a strong blue flame to drive off any moisture on the surface.

When the heating is complete, the crucible and lid are transferred to a desiccator. The desiccator provides a dry atmosphere and allows the crucible and lid to cool without adsorbing a layer of moisture. Once the crucible and lid have cooled to room temperature, they are weighed on an analytical balance which ideally should read to ±0.001 g. For this analysis is important to have very precise readings.

A sample of the hydrated copper sulfate salt is added to the crucible and the contents reweighed with the mass recorded. This is then heated strongly for approximately 10 minutes. This drives off the water molecules of crystallisation from the salt and leaves anhydrous copper sulfate. During the heating process, the lid should partially cover the contents of the crucible.

In this way, the volatile product, i.e. water, can escape. After cooling in a

desiccator (to allow the substance to cool and prevent it from absorbing

moisture), the crucible and contents are weighed once more. The heating,

cooling and weighing are repeated until two consecutive mass readings, differ

by 0.002 g or less. This procedure is known as heating to a constant mass and

is necessary to ensure that the reaction has gone to completion. From the

loss in mass and the initial mass of the sample, the percentage water in the

salt can be calculated which also allows a value of n to be calculated.

(3)

3 The mass has not altered which means that all the water has been driven off.

In normal practice, this takes several attempts.

Results and Calculation

The results and calculation below are for the determination of n in the hydrated copper sulfate salt, CuSO

4

.nH

2

O. Before Heating

Mass of crucible + lid = 20.635g

Mass of crucible + lid + CuSO

4

.nH

2

O = 26.875g Mass of CuSO

4

.nH

2

O = 6.240g

After Heating (first time)

Mass of crucible + lid + CuSO

4

.nH

2

O = 24.625g Mass of CuSO

4

.nH

2

O = 3.990g (24.625g – 20.635g)

After Heating (second time)

Mass of crucible + lid + CuSO

4

.nH

2

O = 24.625g Mass of CuSO

4

.nH

2

O = 3.990g (24.625g – 20.635g) Mass of anhydrous copper sulfate = 3.990g

Mass of water driven off = 2.250g (6.240g – 3.990g)

2. Precipitation

In precipitation methods of gravimetric analysis, a substance is dissolved in

water and converted into an insoluble product by the addition of a suitable

reagent. The resulting precipitate is then filtered and separated from the

filtrate. The filtrate is tested to ensure that the reaction is completed by

adding more reagent to it. Once complete, the precipitate is washed, dried

and finally weighed. By isolating the precipitate and analysing it we use that

(4)

4 substance.

Example B: Determination of nickel using dimethylglyoxime

The nickel content of a nickel(II) salt can be achieved by reacting the nickel(II) ions with dimethylglyoxime.

GFM 288.7

0.968g of an impure sample of nickel(II) sulfate, NiSO

4

.7H

2

O, was dissolved in water and reacted with dimethylglyoxime, forming 0.942g of a red precipitate. Calculate the percentage, by mass, of nickel in the impure sample of nickel (II) sulfate.

The red, nickel(II) dimethylglyoximate complex, is filtered from the

reaction mixture, dried and weighed.

(5)

5 Results and Calculation

Nickel dimethylgloximate (red complex) : Ni 1 mole  1 mole

288.7g  58.7g 1g  58.7 288.7

0.942g  58.7 x 0.942 0.1915g of nickel present 288.7

This also means that there must be 0.1915g of nickel present in the impure hydrated nickel sulfate, NiSO

4

.7H

2

O

% by mass of nickel in the salt:

0.1915 x 100 0.968

19.8%

Example C

5.795g of the precipitate, silver chromate, Ag

2

CrO

4

, was formed by adding excess

potassium chromate, K

2

CrO

4

, to a solution containing silver ions. Calculate the mass

of silver in the solution.

(6)

6 information in the form of an equation.

2Ag

⁺

(aq) + K

2

CrO

4

(aq)  Ag

2

CrO

4

(s) + 2K

⁺

(aq) Ag

2

CrO

4

(s) : 2Ag

⁺

(aq)

1 mole : 2 moles 331.8g  215.8g 1g  215.8 331.8

5.795g  215.8 x 5.795 = 3.77g of silver in the solution.

331.8 Watch the following clips.

https://drive.google.com/file/d/1FM7BIEeq40ZCwUZ9uHr8BB4FAHOlamXE/view?us p=sharing

https://www.youtube.com/watch?v=peMyqdJ57dA https://www.youtube.com/watch?v=X558eJKFaRs

 Read Scholar Heriot-Watt/ Researching Chemistry Section 4.

 Read Bright Red text book pages 80 and 81.

 Answer the questions from Sheet 4.5 and check the answers when you have completed them.

(7)

7 4.5 Gravimetric Analysis

1. 0.7011 g of an impure chloride solution was reacted with AgNO

3

in a precipitation reaction producing 0.9805 g of AgCl.

What was the percentage by mass of chloride in the impure chloride solution?

2. Gunmetal is an alloy of, mainly, copper and tin. The copper content is sufficiently high to be worth recovering from scrap gunmetal. In order to determine the approximate percentage of copper in a sample of gunmetal the following experiment is carried out. 0·500 g of gunmetal was weighed and reacted with nitric acid. This produced a sample of copper (II) nitrate solution and an insoluble tin compound. The copper nitrate solution was added to a solution of sodium carbonate and insoluble copper(II) carbonate was produced. The copper carbonate was placed in a pre-weighed crucible and heated strongly until it had all decomposed to copper(II) oxide.

Results

Mass of crucible + copper(II) oxide = 26·658 g Mass of crucible = 26·101 g

Calculate the percentage of copper in the gunmetal alloy.

3. Nickel(II) ions react quantitatively with dimethylglyoxime (C

4

H

8

O

2

N

2

) forming a complex which precipitates out as a red solid. The equation for the reaction and the structure of the complex are shown below.

Ni

²⁺

+ 2C

4

H

8

O

2

N

2

 Ni(C

4

H

7

O

2

N

2

)

2

+ 2H

⁺

When 1.267 g of an impure sample of nickel(II) sulfate, NiSO

4

.6H

2

O, was dissolved in water and reacted with dimethylglyoxime, 1.035 g of the red precipitate was formed.

a) Calculate the mass, of nickel in the complex (red precipitate) formed.

b) Calculate the total mass of pure hydrated nickel sulfate in the sample.

(8)

8 was dissolved in nitric acid and the resulting solution was diluted to 500 cm

³

in a volumetric flask. A 50 cm

³

portion was removed and treated with 0.2 mol l

^-1

hydrochloric acid producing a precipitate of silver (I) chloride.

The precipitate was recovered by filtration, washed and dried to constant mass and found to have a mass of 0.71 g.

Calculate the percentage by mass of silver in the alloy.

5. 50cm

³

of 0.5 mol l

^-1

AgNO

3

(aq) is reacted with excess calcium chloride. What mass of precipitate forms?

6. In a gravimetric analysis of silver, a precipitate of silver (I) chromate was

produced by adding excess potassium chromate to a solution containing silver (I) ions. If 3.863 g of Ag

2

CrO

4