Certificate Purity

Approximate determination of acid concentrations

1. 1. Introduction

In the chemical laboratory it is common practice to prepare standard solutions of acids for use in titration work. Typically, the volume of acid to be diluted in 1 dm3 of water is usually provided in conjunction with standard equations as well as mass in grams for the base, from which the volume of acid required in neutralizing a known volume of base (alkali) can be determined.

In Nigeria, for instance, the two major examining boards at the secondary level- the West African Examination Council (WAEC) and National Examinations Council (NECO), provide this information to schools preparing to write their chemistry practical exams.

This data are necessary to aid in pre-titration works in the chemistry laboratory in order prepare the students as well as the laboratory assistant for the actual chemistry practical exams. In particular, a very important computation is the determination of the concentration of acid in mol/dm3.If this parameter can be obtained prior to the titration experimentations then it can facilitate the preparation of standard solutions of acids and of course ensure accuracy in post- titration work.

This paper is structured as follows:

In section 2 the required concentration of acids is computed from first principles. In section 3 we compute the acid concentration using a single equation.

Finally, we give our conclusions.

 

 

1. 2. Computation of Acid Concentrations from First Principles

In the preparation of the standard solution of an acid certain parameters must as a rule be specified. These parameters are:

(i)               the specific gravity (s.g), in g/cm3 per g/cm3 (ρ/ρ)

(ii)             the percentage purity (% P) in %

(iii)           the relative molar mass (Mr), in g/mol

(iv)           the required volume of concentrated acid to be diluted to 1dm3 of distilled water (Vr) in cm3

The next stage is to specify their values for use in titration analysis. The table below presents approximate values of M, %P and s.g of some common acids. [1], [2]:

s/n

Acid

Mr (g/mol)

%P (Mr/Mr)

s.g (ρ/ρ)

1

HNO3

63.0

65-72

1.047-1.052

2

HCL

36.5

32-36

1.18-1.19

3

H2SO4

98.0

96-98

1.84

 

Table1 Percentage Purity, specific gravity and molar mass of some commonly used acids.

From the theory of simple proportions [3]:

1 cm3 of HNO3 contain 1.5 g of HNO3

1000cm3 (1dm3) of HNO3 will contain 1500g of HNO3 which corresponds to impure mass of HNO3 in 1dm3 of distilled water.

This is for the impure case. For the pure case, we specify the percentage purity of the impure acid (HNO3 in this case), in 1dm3 of distilled water:

%P = 70% for HNO3,

Pure mass of HNO3, mp = 70% of 1500g = 1050g

The amount of pure HNO3 (Cp) is determined next from the relation:

Cp = mp/Mr ——- (1)

For HNO3, Mr = 63g, hence,

Cp = 16.67 mol/dm3.

Finally, applying the dilution formula,

Cp * V = Cd * Vr ——— (2)

 

From which,

Cd = (Cp * Vr) / V —— (3)

Where V = 1000cm3

Cd = desired concentration in mol/dm3 for Vr = 5.4 cm3.

 

1. 3. Computation of Acid Concentrations from Formula

A suitable formula has been developed and is presented below:

Cd = (%P * s.g * Vr) / (100 * Mr) —— (4)

This formula is derived from the dilution formula and eliminates the usual steps required in the analysis of a standard solution of an acid. Substituting previous parameter values as for HNO3 acid would result in same results for Cd, the required dilute concentration of the acid. Other Cd values for different acids can be similarly obtained by using formula methods (see equation 4).

1. 4. Conclusion

Inside this work, we have shown that the required dilute concentration of an acid can be obtained from first principles using the dilution formula.

We have also shown this can be done using a single equation and this can help facilitate the laboratory process.

We propose a computerized program in excel or other software such as visual basic in respect to the above formula.

 

References:

1. Chendo M.N., Comprehensive Practical Chemistry, Hybrid Publishers Ltd, 1994, Nigeria, 39-41.
2. A. Holderness, J. Lambert, A New Certificate Chemistry, Heinemann Educational Books, 6th ed , Nigeria, 131-136.
3. I.O. Uche, I.J Adenuga and S.L Iwuagwu, Countdown to SSCE/JUNE Chemistry, Evans Brothers (Nigeria Publishers) Limited, 1990, 298-300.

About the Author

Director Research and Development, J.C. JOEMATECH RESOURCES NIG. LTD

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