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In an experiment to measure the concentration of dissolved oxygen in a river water sample, a bottle of water was filled from the river and it was analysed immediately - Leaving Cert Chemistry - Question 1 - 2005
Question 1
In an experiment to measure the concentration of dissolved oxygen in a river water sample, a bottle of water was filled from the river and it was analysed immediatel... show full transcript
Worked Solution & Example Answer:In an experiment to measure the concentration of dissolved oxygen in a river water sample, a bottle of water was filled from the river and it was analysed immediately - Leaving Cert Chemistry - Question 1 - 2005
Step 1
Why is it necessary to make the additions to the sample water immediately?
Answer
The additions must be made immediately to prevent any changes in the oxygen content due to biological activities such as respiration by microorganisms. Delays could result in an increase in the oxygen content as microorganisms may consume the dissolved oxygen, leading to inaccurate measurements.
Step 2
Why should the solutions used be concentrated?
Answer
Using concentrated solutions minimizes the dilution effect on the original water sample ensuring that the changes to the oxygen levels due to added solutions are negligible. It also helps to increase the sensitivity of detection when forming the colored complex.
Step 3
Describe how to ensure that any water overflow from the bottle/addition made so that water overflows from the bottle
Answer
To ensure that water overflows from the bottle, hold the bottle upright while adding the reagents slowly. This prevents air from entering the bottle and allows for an overflow that avoids disrupting the entire sample.
Step 4
Describe how to avoid bubbles of air (oxygen) entering the water in the process
Answer
To avoid air bubbles, carefully introduce the syringe or pipette to the water sample at an angle and add the solution slowly. Do not squirt the liquid into the bottle rapidly as this can cause air to mix with the water.
Step 5
How to rinse your pipette to ensure exactly 50 cm³ portions of the iodine are delivered to the titration flask?
Answer
First, use the solution you are going to titrate with to rinse the pipette. Draw up the solution into the pipette and then discard it, ensuring that residual solution does not contaminate the iodine sample. Repeat this process two to three times before filling the pipette for the final measurement.
Step 6
Step 7
Calculate the concentration of the iodine solution in moles per litre
Answer
Using the volume of sodium thiosulfate (6.0 cm³ or 0.006 L) and its concentration (0.01 M), we can calculate:
Next, if 2 moles of iodine react with 2 moles of thiosulfate: ext{Concentration of I} = rac{6.0 imes 10^{-5} ext{ moles}}{0.0060 ext{ L}} = 0.01 ext{ M}
Step 8
Calculate the concentration of the dissolved oxygen in the river sample in p.p.m.
Answer
Given that 0.0006 g of oxygen is present in 6.0 cm³ (or 0.006 L) of water, we calculate:
ext{p.p.m.} = rac{ ext{mass of solute (mg)}}{ ext{volume of solution (L)}} = rac{0.0006 ext{ g} imes 1000 ext{ mg/g}}{0.006 ext{ L}} = 100 ext{ p.p.m.}
Thus, the concentration of dissolved oxygen is 9.6 p.p.m.