Question 1

In the Winkler Method, what is the purpose of treating the water sample with manganous sulfate and potassium iodide?

Accepted Answer

To determine the concentration of dissolved oxygen

Answer

To oxidize Mn2+ to Mn3+

Answer

To titrate with sodium thiosulfate

Answer

To convert manganic hydroxide to manganic sulfate

Question 2

What is the chemical equation for the addition of MnSO4 and alkali-iodide-sodium azide in the water sample?

Accepted Answer

MnSO4 + alkali-iodide-sodium azide -> Mn2(SO4)3 + I3-

Answer

MnSO4 + alkali-iodide-sodium azide -> Mn2(SO4)3 + I-

Answer

MnSO4 + alkali-iodide-sodium azide -> Mn3+ + I3-

Answer

MnSO4 + alkali-iodide-sodium azide -> Mn(OH)3 + I2

Question 3

What is the chemical identity of the white precipitate formed after the addition of MnSO4 and alkali-iodide-sodium azide in the water sample?

Accepted Answer

Manganic hydroxide

Answer

Manganous sulfate

Answer

Triiodide

Answer

Manganic sulfate

Question 4

Why do you need to titrate until the solution turns pale yellow first before adding the starch indicator?

Accepted Answer

To ensure complete dissolution of the precipitate

Answer

To determine the concentration of dissolved oxygen

Answer

To neutralize the acid in the solution

Answer

To prevent interference with the starch indicator

Question 5

What is the purpose of adding concentrated H2SO4 to the water sample after the precipitates have settled?

Accepted Answer

To convert manganic hydroxide to manganic sulfate

Answer

To titrate with sodium thiosulfate

Answer

To oxidize iodide to triiodide

Answer

To neutralize the solution

Question 6

What is the purpose of allowing the bottle to stand for 10 minutes in the dark after adding concentrated H2SO4?

Accepted Answer

To stabilize the solution

Answer

To allow the precipitate to settle

Answer

To prevent interference with the titration

Answer

To ensure complete dissolution of the precipitate

Question 7

What is the concentration of the Na2S2O3 solution used for titration?

Accepted Answer

0.025 N

Answer

0.05 N

Answer

0.2 N

Answer

0.1 N

Question 8

What is the purpose of adding starch solution to the titration flask?

Accepted Answer

To indicate the endpoint of the titration

Answer

To neutralize the solution

Answer

To determine the concentration of dissolved oxygen

Answer

To prevent interference with the Na2S2O3 titration

Question 9

How should the Na2S2O3 be titrated in relation to the solution turning pale yellow?

Accepted Answer

Titrate until the solution turns pale yellow

Answer

Titrate until the solution turns blue

Answer

Titrate until the solution turns green

Answer

Titrate until the solution turns colorless

Question 10

What is the relationship between the quality of water and the amount of dissolved oxygen present in the sample?

Accepted Answer

Higher quality water has higher dissolved oxygen levels

Answer

Lower quality water has higher dissolved oxygen levels

Answer

There is no relationship between water quality and dissolved oxygen levels

Answer

The relationship between water quality and dissolved oxygen levels is unpredictable

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