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In an experiment to measure the dissolved oxygen content of a river water sample, a small amount of a concentrated solution of compound A, followed by a small amount of a concentrated solution of alkaline potassium iodide (KOH/KI), were added to a bottle filled with the river water - Leaving Cert Chemistry - Question 1 - 2013
Question 1
In an experiment to measure the dissolved oxygen content of a river water sample, a small amount of a concentrated solution of compound A, followed by a small amount... show full transcript
Worked Solution & Example Answer:In an experiment to measure the dissolved oxygen content of a river water sample, a small amount of a concentrated solution of compound A, followed by a small amount of a concentrated solution of alkaline potassium iodide (KOH/KI), were added to a bottle filled with the river water - Leaving Cert Chemistry - Question 1 - 2013
Step 1
Why was it important to avoid trapping air bubbles each time the stopper was inserted into the sample bottle when using the dropper?
Answer
It is crucial to avoid trapping air bubbles because they can introduce atmospheric oxygen into the sample, which would lead to inaccurate measurements of the dissolved oxygen content. Maintaining an anaerobic environment ensures that the test only measures the oxygen that is genuinely dissolved in the water.
Step 2
Step 3
What was observed on the addition of the concentrated sulfuric acid (H₂SO₄) to the sample contents of the bottle?
Answer
Upon adding concentrated sulfuric acid (H₂SO₄), a brown color develops in the solution due to the formation of iodine from the reaction between the iodine and the sulfuric acid. This indicates a change in oxidation states and formation of iodine, which is visually noticeable.
Step 4
Describe how the burette was rinsed and filled for use in the titrations.
Answer
To prepare the burette, it was first rinsed with a small amount of the sodium thiosulphate solution to ensure that no contaminants influence the titration result. The burette was then filled with the thiosulphate solution, ensuring that it was vertical for accurate reading.
Step 5
Step 6
Calculate the concentration of iodine (I₂) in the sample bottle in moles per litre.
Answer
To calculate the concentration of I₂, first, use the titration data. Knowing that 9.4 cm³ of 0.02 M sodium thiosulphate was required:
- Calculate moles of Na₂S₂O₃ used:
- From the balanced equation, the molar ratio of I₂ to Na₂S₂O₃ is 1:5, thus: ext{Moles of I₂} = rac{0.000188}{5} = 0.0000376 ext{ mol}
- The concentration of I₂ in 200 cm³ (0.2 L) is: ext{Concentration} = rac{0.0000376}{0.2} = 0.000188 ext{ mol/L}
Step 7
Give the formula for dissolved oxygen (O₂) in the water sample, two moles of iodine (I₂) are liberated from the whole experiment. Calculate the concentration of dissolved oxygen in the river water.
Answer
To find the concentration of dissolved oxygen (O₂), one must realize that:
- The molar ratio of O₂ to I₂ produced is typically 1:2. Hence, for every mole of O₂, 2 moles of I₂ are needed.
- Given that for 0.0000376 mol of I₂, we have: ext{Moles of O₂} = rac{0.0000376}{2} = 0.0000188 ext{ mol}
- The concentration of dissolved oxygen is calculated as follows: ext{Concentration} = rac{0.0000188 ext{ mol}}{0.2 ext{ L}} = 0.000094 ext{ mol/L}
- To convert to parts per million (ppm), assuming that 1 mol of O₂ weighs 32 g: ext{ppm} = rac{(0.000094 ext{ mol/L}) imes 32 ext{ g/mol}}{1 ext{ L}} imes 10^6 = 3.008 ext{ ppm}
Step 8
State how it would have been reached had a white precipitate been observed instead of the brown precipitate during the distillation process of reagents to a bottle filled with river water?
Answer
If a white precipitate was observed instead of the expected brown precipitate, it would indicate a possible contamination or an initial reaction that was not anticipated. This might suggest the presence of different ions or compounds within the river water sample, potentially altering the expected chemical processes of the experiment.
Step 9
Describe how temperature would affect the determination of dissolved oxygen (O₂) considered best practice?
Answer
Temperature has a significant effect on the solubility of gases in liquids. As temperature increases, the solubility of dissolved oxygen decreases, which means that warmer water holds less oxygen. Best practice dictates that the determination of dissolved oxygen should be conducted at a standard temperature, preferably at 20°C, to ensure consistency and comparability of results.