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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 was important to avoid trapping air bubbles to prevent the introduction of atmospheric oxygen into the sample. The presence of air bubbles could affect the accuracy of the dissolved oxygen measurement by increasing the oxygen levels artificially, which would lead to incorrect results in the experiment.
Step 3
What was observed on addition of the concentrated solution followed by the sulfuric acid?
Answer
The addition of the concentrated solution followed by sulfuric acid resulted in a browning of the solution, which indicated the presence of iodine, forming a brown precipitate due to the reaction taking place.
Step 4
Describe how the burette was rinsed and filled for use in the titrations.
Answer
The burette was rinsed with deionized water followed by a rinse with a small amount of the sodium thiosulfate solution. The stopper was then removed, and the burette was filled with the thiosulfate solution while ensuring it was vertical. Care was taken to avoid air bubbles, and the burette tip was filled to ensure correct titration results.
Step 5
Step 6
Calculate the concentration of dissolved oxygen (O₂) in the sample bottle in moles per litre.
Answer
The average titre is 9.4 cm³. The concentration of sodium thiosulfate is 0.02 mol/L. The number of moles of thiosulfate used can be calculated as:
Since 1 mole of iodine reacts with 2 moles of thiosulfate, the moles of O₂ is half:
n(O_2) = rac{1.88 imes 10^{-4}}{2} = 9.4 imes 10^{-5} ext{ mol}
The total volume of the sample bottle is 1 dm³, leading to a concentration of:
C(O_2) = rac{9.4 imes 10^{-5} ext{ mol}}{1 ext{ L}} = 9.4 imes 10^{-5} ext{ mol/L}
Step 7
Write the value of dissolved oxygen (O₂) in the water sample, two modes of (1) are given for you. Explain how to calculate the concentration of dissolved oxygen in the river water:
Answer
To convert the concentration of dissolved oxygen from moles per litre to grams per litre, the formula used is:
The molar mass of oxygen (O₂) is approximately 32 g/mol. Therefore:
Step 8
Why, given that the end point has been reached had a white precipitate been observed instead of the brown (red, orange, yellow, iodine) solution?
Answer
A white precipitate indicates the formation of a starch-iodine complex owing to excess iodine being present. Upon reaching the endpoint, the iodine is consumed but a precipitate can form if there is inadequate thiosulfate, leading to unclear results not relating directly to the original solution.
Step 9
Briefly, describe what was done to the sample bottle from when the titration began until the end was reached. Why is the immediate determination of dissolved oxygen important?
Answer
During the titration, the sample bottle was thoroughly mixed with the added reagents before titration began. The titration process included careful addition of sodium thiosulfate until the indicator changed color, signifying that the endpoint had been reached. Immediate determination of dissolved oxygen is critical because O₂ levels can change rapidly due to biological activity in water samples, thereby affecting the accuracy of measurements.