10-minute Quiz : Chapter 63 Quantitative methods of analysis

Direction: Questions 1 and 2 refer to the experimental determination of iron content in a brand of commercial iron tablets.

2.024 g of ground iron tablets was dissolved in excess dilute sulphuric acid. Then the tablet solution was transferred to a 100.0 cm³ volumetric flask and made up to a solution of 100.0 cm³ by adding distilled water. After that, 10.0 cm³ portions of the acidified iron tablet solution were titrated against a standard potassium permanganate solution. The average amount of iron(II) ions in these portions was found to be 5.38 × 10⁻⁵ mol. What is the percentage by mass of iron in the iron tablets?

(Relative atomic mass: Fe = 55.8)

• A
• B
• C
• D

Direction: Questions 1 and 2 refer to the experimental determination of iron content in a brand of commercial iron tablets.

What type of titration was involved in the above volumetric analysis?

• A. Acid-alkali titration
• B. Acid-carbonate titration
• C. Redox titration
• D. Precipitation titration

The concentration of an acid was known to be 0.101 M. A student determined the concentration of the acid three times, and the respective results were 0.088 M, 0.087 M, and 0.089 M. Which of the following comments on these results is correct?

• A. The results were accurate but not precise.
• B. The results were precise but not accurate.
• C. The results were neither accurate nor precise.
• D. The results were accurate and precise.

Direction: Questions 4 to 6 refer to the experimental determination of concentration of hydrochloric acid in a brand of brick cleaner.

A 10.0 cm³ sample was withdrawn from the bottle of brick cleaner. Then the sample was diluted to 250.0 cm³ with distilled water. After that, 25.0 cm³ portions of the diluted brick cleaner sample were titrated with a 0.05342 M sodium carbonate solution, using methyl orange as an indicator. The mean volume of the sodium carbonate solution used to reach the end point was found to be 20.40 cm³. What is the concentration of hydrochloric acid in the 10.0 cm³ sample of brick cleaner?

• A. 0.0109 M
• B. 0.0164 M
• C. 2.18 M
• D. 5.45 M

Direction: Questions 4 to 6 refer to the experimental determination of concentration of hydrochloric acid in a brand of brick cleaner.

The 0.05342 M sodium carbonate solution mentioned above can be prepared by dissolving anhydrous sodium carbonate in distilled water. What is the mass of the anhydrous sodium carbonate required to prepare 100.0 cm³ of this standard solution?

(Relative atomic masses: C = 12.0, O = 16.0, Na = 23.0)

• A. 0.0504 g
• B. 0.5342 g
• C. 0.5350 g
• D. 0.5663 g

Direction: Questions 4 to 6 refer to the experimental determination of concentration of hydrochloric acid in a brand of brick cleaner.

The hydrochloric acid concentration determined experimentally was found to be lower than that claimed by the manufacturer of this brand of brick cleaner. Which of the following could reasonably account for the difference?

• A. Some hydrogen chloride may have evaporated from the sample during the analysis.
• B. The brick cleaner may have absorbed carbon dioxide from the air.
• C. The anhydrous sodium carbonate used for preparing the standard solution was impure.
• D. The burette was rinsed with water only.

In an experiment to standardize a bottle of 1000 cm³ sodium hypochlorite solution, 10.0 cm³ of the solution sample was diluted to 100.0 cm³ by using distilled water. Then excess potassium iodide solution and excess sulphuric acid were added to a 25.0 cm³ of diluted solution sample to convert all the hypochlorite ions to iodine.

OCl⁻(aq) + 2H⁺(aq) + 2I⁻(aq) → I₂(aq) + Cl⁻(aq) + H₂O(l)

The resultant solution was titrated with 0.0500 M sodium thiosulphate solution using starch solution as an indicator. The titration was repeated several times and the mean volume of the sodium thiosulphate solution required to reach the end point was found to be 27.40 cm³. What is the concentration of the bottle of sodium hypochlorite solution?

I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)

• A. 2.74 × 10⁻³ M
• B. 6.85 × 10⁻³ M
• C. 0.274 M
• D. 6.85 M

Direction: Questions 8 and 9 refer to the experimental determination of the mass of ascorbic acid, also known as vitamin C, in a brand of vitamin C tablets.

Dissolve 2.05 g of a vitamin C tablet in deionized water. Then transfer the tablet solution to a 1000.0 cm³ volumetric flask and make up to a 1000.0 cm³ solution with distilled water. After that, titrate 25.0 cm³ portions of the vitamin C tablet solution against a standardized 0.0100 M iodine solution using a suitable indicator. The mean volume of the 0.0100 M iodine solution required to reach the titration end point is found to be 23.70 cm³. What is the mass of ascorbic acid in the vitamin C tablet?

C₆H₈O₆(aq) + I₂(aq) → C₆H₆O₆(aq) + 2H⁺(aq) + 2I⁻(aq)

(Relative molecular mass: ascorbic acid = 176.0)

• A. 1.24 g
• B. 1.43 g
• C. 1.52 g
• D. 1.67 g

Direction: Questions 8 and 9 refer to the experimental determination of the mass of ascorbic acid, also known as vitamin C, in a brand of vitamin C tablets.

What indicator should be used for this titration?

• A. Phenolphthalein
• B. Starch solution
• C. Potassium chromate solution
• D. Potassium permanganate solution

In an experiment, 6.00 g of a sample of hydrated barium chloride (BaCl₂•2H₂O) was dissolved in deionized water and made up to a 250.0 cm³ solution with deionized water. Then a titration was carried out using a standard silver nitrate solution with a suitable indicator. The ionic equation for the reaction involved in the titration is provided below.

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

25.0 cm³ of the barium chloride solution required 18.60 cm³ of 0.25 M silver nitrate solution to reach the titration end point. What is the purity, in terms of percentage by mass, of this hydrated barium chloride sample?

(Relative atomic masses: H = 1.0, O = 16.0, Cl = 35.5, Ba = 137.3)

• A. 71.8%
• B. 84.7%
• C. 91.4%
• D. 94.7%