Q: A student titrates 0.0500 M KIO3 (25.00 mL) with sodium thiosulfate. If 15.00 mL of thiosulfate is required to reach the endpoint, what is the molarity of the thiosulfate solution? (Use the stoichiometry: $$6\mathrm{S}_2\mathrm{O}_3$$^{2-}$$ per 1$$\mathrm{IO}_3^-$$)

$$0.0500 \times \frac{0.02500}{0.01500} \times \frac{1}{6} = 0.0139\ \mathrm{M}$$

Question 1

Which of the following best describes the role of starch in the redox titration of hypochlorite in bleach?

Accepted Answer

Starch forms a colored complex with I2, allowing visual detection of the endpoint.

Question 2

In the standardization of sodium thiosulfate using potassium iodate, which of the following equations correctly represents the overall reaction?

Accepted Answer

$$6\mathrm{S}_2\mathrm{O}_3$$^{2-}$$(aq) + \mathrm{IO}_3^-(aq) + 6\mathrm{H}^+(aq) \rightarrow 3\mathrm{S}_4\mathrm{O}_6$$^{2-}$$(aq) + \mathrm{I}^-(aq) + 3\mathrm{H}_2\mathrm{O}(l)$$

Question 3

A student titrates 0.0500 M KIO3 (25.00 mL) with sodium thiosulfate. If 15.00 mL of thiosulfate is required to reach the endpoint, what is the molarity of the thiosulfate solution? (Use the stoichiometry: $$6\mathrm{S}_2\mathrm{O}_3$$^{2-}$$ per 1$$\mathrm{IO}_3^-$$)

Accepted Answer

$$0.0500 	imes \frac{0.02500}{0.01500} 	imes \frac{1}{6} = 0.0139\ \mathrm{M}$$

Question 4

During the titration of bleach, which species is directly titrated with thiosulfate?

Accepted Answer

I$$_2$$

Question 5

If a student uses 0.00200 mol of sodium thiosulfate to titrate a bleach sample, how many moles of hypochlorite ion (OCl$$^-) $$were present in the sample? (Use the stoichiometry: $$2\mathrm{S}_2\mathrm{O}_3$$^{2-}$$ per 1$$\mathrm{OCl}^-$$)

Accepted Answer

$$0.00100\ \mathrm{mol}$$

Question 6

Which of the following best explains why iodide (I$$^-) is $$added during the titration process?

Accepted Answer

Iodide is oxidized by hypochlorite or iodate to form I$$_2$$, which is then titrated with thiosulfate.

Question 7

A bleach sample is titrated and found to contain 0.00500 g of OCl$$^-. If $$the mass of the bleach sample is 0.200 g, what is the mass percent of hypochlorite in the bleach?

Accepted Answer

$$2.5\%$$

Question 8

Which of the following color changes correctly indicates the endpoint in the titration of I$$_2$$ with thiosulfate using starch as an indicator?

Accepted Answer

Deep blue to colorless

Question 9

Why is it important to standardize the sodium thiosulfate solution before using it to titrate bleach?

Accepted Answer

To determine the exact molarity of thiosulfate for accurate calculation of hypochlorite content

Question 10

A student accidentally adds starch at the very beginning of the titration, before any thiosulfate is added. What is the most likely consequence?

Accepted Answer

The blue color may become too intense and persist, making the endpoint difficult to observe.

Question 11

Which of the following best describes the function of sulfuric acid in the standardization titration involving iodate and thiosulfate?

Accepted Answer

It provides the acidic environment necessary for the oxidation of iodide to iodine by iodate.

Question 12

In the titration of bleach, which of the following correctly represents the stoichiometric relationship between thiosulfate and hypochlorite?

Accepted Answer

$$2\ \mathrm{mol}\ \mathrm{S}_2\mathrm{O}_3$$^{2-}$$ : 1\ \mathrm{mol}\ \mathrm{OCl}^-$$

Question 13

A student prepares 25.00 mL of 0.0500 M KIO3. What mass of KIO3 (molar mass = 214.0 g/mol) is required?

Accepted Answer

$$0.2675\ \mathrm{g}$$

Question 14

Which of the following is a correct sequence of color changes observed during the titration of I$$_2$$ with thiosulfate in the presence of starch?

Accepted Answer

Reddish-brown → pale yellow → deep blue → colorless

Question 15

If a student uses 0.00150 mol of thiosulfate to titrate a bleach sample, and the molar mass of OCl$$^- is 51.45 g/$$mol, what mass of OCl$$^- is $$present in the sample? (Use the stoichiometry: $$2\mathrm{S}_2\mathrm{O}_3$$^{2-}$$ per 1$$\mathrm{OCl}^-$$)

Accepted Answer

$$0.0386\ \mathrm{g}$$

Question 16

Which of the following statements about the titration of hypochlorite in bleach is correct?

Accepted Answer

Iodide is oxidized by hypochlorite to form I$$_2$$, which is then titrated with thiosulfate.

Question 17

A student titrates 0.100 g of bleach and finds it contains 0.00200 mol of OCl$$^-. $$What is the mass percent of OCl$$^- in $$the bleach? (Molar mass of OCl$$^- = 51.45 g/$$mol)

Accepted Answer

$$102.9\%$$

Question 18

Which of the following best explains why the solution becomes colorless at the endpoint of the titration?

Accepted Answer

All I$$_2$$ has been reduced to I$$^-$$, so the starch-I$$_2$$ complex is no longer present.

Redox Titration: Determination of Hypochlorite in Bleach Using Sodium Thiosulfate

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