Chem 0100 Moles, Stoichiometry, Atomic Structure

Aspirin was prepared by reacting 4.0 g of salicylic acid and 6.0 mL of acetic anhydride, using 0.5 mL of sulfuric acid as a catalyst. The percentage yield was 92%. Calculate the actual mass of aspirin obtained from this experiment?

• 5.2 g

• 4.8 g

• 48 g

• 52 g

State two methods that can be used to confirm the purity of aspirin obtained by reaction of salicylic acid, acetic anhydride and sulfuric acid?

• The melting point range of pure aspirin is 138-140^oC.

• Fractional distillation.

• Using iron(III) chloride.

• Using ethanol.

In an experiment, 1.5766 g of an ammonium salt was placed in a beaker and 60 mL of 1.181 M sodium hydroxide was added. The resulting mixture was heated for about an hour and then left to cool to room temperature. The reaction mixture was then titrated with sulfuric acid and required 18.35 mL of the acid for complete neutralization. Given that the percentage by mass of ammonium in the salt was 37.5 %, calculate the molarity of the sulfuric acid used in the titration.

• 1.038 M

• 1.114 M

• 2.075 M

• 2.228 M

• n = 3, 2.93 x 10¹⁵ s^-1

• n = 2, 2.74 x 10¹⁴ s^-1

• n = 6, 2.74 x 10¹⁴ s^-1

• n = 1, 2.93 x 10¹⁵ s^-1

What is the concentration of a dilute solution of sulphuric acid if an average titre volume of 22.50 cm^3 of sulphuric acid is needed to neuralise a 25.00 mL aliquot of stock solution made by transferring 4.805 g of solid sodium tetraborate (Na2B4O7) to a 250 mL volumetric flask, and the solution made up to the mark with distilled water. Bromocresol green was used as an indicator. When phenolphtalein was used as indicator the average volume of sulphuric acid needed was 9.50 cm^3.

• 0.11 M when bromocresol green was used.

• 0.03 M when bromocresol green was used.

• 1.13 M when bromocresol green was used

• 2.04 M when bromocresol green was used.

If an average titre volume of 22.50 cm^3 of sulphuric acid is needed to neuralise a 25.00 mL aliquot of stock solution made by transferring 4.805 g of solid sodium tetraborate (Na2B4O7) to a 250 mL volumetric flask, and the solution made up to the mark with distilled water when bromocresol green was used as an indicator. And when phenolphtalein was used as indicator the average volume of sulphuric acid needed was 9.50 cm^3. Account for the difference in the titre values for both indicators, and hence explain why the titre obtained with phenolphthalein is unacceptable. pH range of bromocresol green is 3.8 - 5.4. pH range of phenolphthalein is 8.3 - 10.

• Titration is between a weak acid and strong base so the pH at the end point is close to 9. As acid is added the phenolphtalein will not change colour when the end point is reached.

• Titration is between a weak acid and weak base so the pH at the end point is close to 7. As acid is added the phenolphtalein will change colour before the end point is reached.

• Titration is between a strong acid and weak base so the pH at the end point is close to 5. As acid is added the phenolphtalein will change colour before the end point is reached.

• Titration is between a strong acid and strong base so the pH at the end point is close to 7. As acid is added the phenolphtalein will change colour before the end point is reached.

Why is sodium hydroxide not considered a primary standard?

• Sodium hydroxide is hygroscopic, that is, it attracts moisture from the air. Hence the concentration of its solution varies and is not reliable.

• Sodium hydroxide can be used as a primary standard in titrations.

• Sodium hydroxide absorbs carbon dioxide from the air which will make it impure and affect the formula.

• Sodium hydroxide acts as a base and can be used to neutralize an acid.