Acid Base Titrations Lab 5y2q45

ACID - BASE TITRATIONS Objectives: At the end of this session you should be familiar with the following: i) ii) iii) iv) v)

The principles upon which volumetric analyses are based. The characteristics of primary standard solution. The theory of acid-base indicators. The mole concept. The principles involved in calculations in volumetric analysis.

Task: In today's laboratory session you are required to perform the following: 1. Prepare a standard solution of anhydrous sodium carbonate. 2. Standardize a solution of hydrochloric acid using the sodium carbonate solution in A. 3. Standardize a solution of sodium hydroxide using the hydrochloric acid in B. 4. Determine the relative molecular mass of an acid (solid sample) using the sodium hydroxide in C. 5. Determine the thiamine content in the tablets.

EXPERIMENT 1

A.

STANDARDIZATION OF HYDROCHLORIC ACID (USING SODIUM CARBONATE)

Preparation of a Standard Solution of Sodium Carbonate

Procedure: Weigh out accurately, approximately 1.3 g. of anhydrous sodium carbonate into a volumetric flask (250 mL). To do this, first weigh out approximately 1.3 g of the sodium carbonate in a sample bottle using the top loading balance provided. Then using the analytical balance weigh accurately to four decimal places, sample bottle and contents. Transfer quantitatively into volumetric flask by decanting small amounts of sample from bottle into a funnel and washing carefully into flask with distilled water. Shake to completely dissolve the sodium carbonate and then carefully make up to the graduation mark with distilled water. Re-weigh sample bottle, record all mass in your note-book and determine mass of sodium carbonate used by the method of difference.

10

Results

B.

Mass of bottle and salt

=

Mass of bottle

=

Mass of salt

=

Standardization of Hydrochloric Acid

Procedure: Pipette 25 mL aliquots of the standard sodium carbonate solution prepared in Exercise A above into conical flasks (250 mL). Titrate these samples with the hydrochloric acid in the burette using methyl orange as indicator. Repeat until concordant results are obtained. Record your results in tabular form in your note-book, as indicated below.

BURETTE READINGS Final volume (mL)

ROUGH

1

11

Initial volume (mL) Volume acid used

EXPERIMENT 2

A.

DETERMINATION OF THE RELATIVE MOLECULAR MASS OF AN ACID

Standardization of Sodium Hydroxide using Hydrochloric Acid

Procedure: Pipette 25 mL aliquots of the HCl used in Exercise B into conical flasks (250 mL). Fill a clean burette with the NaOH provided. Titrate the samples using phenolphthalein as indicator. Repeat until concordant results are obtained. Record your results in tabular form in your note book. (Why is there need to standardize NaOH?)

11

B.

Determination of the Relative Molecular Mass of an Acid (Solid Sample)

Procedure: Weigh out accurately about 125 mg of the solid acid sample provided into a conical flask (250 mL). [Record the sample number in your notebook and report sheet]. Dissolve the sample in about 50 mL distilled water. Titrate this solution with the sodium hydroxide (from A) in your burette using phenolphthalein as indicator. Do this in duplicate. Record the results in your note book. Wash your burette thoroughly with water after using sodium hydroxide in it. Why is this necessary?

EXPERIMENT 3

DETERMINATION OF THIAMINE CONTENT OF GIVEN SAMPLE TABLETS

Procedure: 1) 2) 3) 4) 5)

Accurately determine mass of tablets. Crush tablets and transfer quantitatively into a conical flask. Add 25 mL distilled water, and shake. Allow mixture to stand for 15 minutes. Add six drops bromothymol blue indicator and titrate with standardized sodium hydroxide.

Colour change: Yellow to bluish green end point 1 mole thiamine hydrochloride (C12H17ClN4OS.HCl) : 1 mole NaOH REPORTING THE RESULTS From Experiment 1, calculate: (i) Number of moles Na2CO3 in 25 mL solution (ii) The molarity of the Na2CO3 solution (iii) The concentration of HCl solution From Experiment 2, calculate: (i) The number of moles NaOH used in the titration (ii) the number of moles NaOH used in titrations against each separate sample of the solid acid. Assuming that this acid is monobasic, calculate: (i) Number of moles of monobasic acid used in each separate sample (ii) Average molar mass of this monobasic acid From Experiment 3, calculate the thiamine content in the sample tablets.

12