Hello everyone
Can please anyone help me with this pre-lab's questions
experiment name 'Dissolved Oxygen in Natural waters
1. For a natural water sample, what range of dissolved oxygen concentrations may you expect?
2. How does the dissolved oxygen concentration in a water sample change (if at all) with
a) ambient temperature changes?
b) atmospheric pressure changes?
c) the volume of the flask collecting the water sample?
d) the amount of organic matter in the water sample?
e) the depth of the body of the water (e.g. lake, river, or ocean)?
3. A solution of MnSO4 is added to fix the dissolved oxygen in the collected sample.
a) What is the meaning of the expression, "fix the dissolved oxygen," and why is it so important for the analysis of dissolved oxygen in a water sample?
b) Only an approximate volume (approximately 1 mL) of MnSO4 is required for fixing the dissolved oxygen in the sample. Explain why an exact volume is not critical.
4. A water chemist obtained a 250-mL sample from a nearby lake and fixed the oxygen on-site with alkaline solutions of MnSO4 and KI-NaN3. Returning to the laboratory, a 200-mL sample was analyzed by acidifying the sample with conc H2SO4 and then titrating with 14.4 mL of 0.0213 M Na2S2O3 solution to the starch endpoint.
a) Calculate the number of moles of I3 that reacted with the Na2S2O3 solution.
b) Calculate the number of moles of Mn(OH)3 that were produced from the reduction of the dissolved oxygen.
c) Calculate the number of moles and milligrams of O2 present in the titrated sample.
d) What is the dissolved oxygen concentration in the sample, expressed in ppm O2?
5. a) What is the procedure for preparing 250 mL of 0.0210 M Na2S2O3 for this experiment from a 100-mL volume of standard 0.106 M Na2S2O3?
b) For the preparation of the 0.0210 M solution in a 250-mL volumetric flask, only a 25.0 mL calibrated volumetric pipet is available. Explain how you would prepare the 0.0210 M Na2S2O3 solution using the 25.0 mL pipet. What would be its exact molar concentration?
thanks in advance