Nearly 800 mL of boiled, deionized water cooled to room temperature is required

Question

Nearly 800 mL of boiled, deionized water cooled to room temperature is required for this experiment. Begin preparing this at the beginning of the laboratory period. Be aware of the number of significant figures when recording data. Prepare a primary standard solution. with your instructor this solution may have already been prepared by stockroom personnel) Use weighing paper (Figure 29.3) to measure the mass (plusminus 0.001 g) of KIO_3 Dissolve the solid KIO_3 and dilate to volume with the freshly boiled, deionized water in a 100-mL volume flask. Calculate and record its exact molar concentration. This solution may have already been prepared by stockroom personnel). This solution should be prepared no more than 1 week in advance because unavoidable decomposition occurs. Measure the mass (plusminus 0.01) of Na_2S_2O_35H_2O with the freshly boiled, deionized water and dilute of 250 mL; an Erlenmeyer flask can be used to prepare and store the solution. Agitate until the salt dissolves. Prepare a buret. Prepare a clean 50-mL buret for titration. Rinse the clean buret with two or three 5-mL portions of the Na_2S_2O_3 solution, draining each rinse through the buret tip. Fill the buret with the NA_2S_2O_3 solution, draining each rinse through the buret tip. fill the buret with the Na_2S_2O_3 solution, drain the tip of air bubbles, and after 10-15 seconds record the volume using all certain digits (from the labeled calibration marks on the buret) plus one uncertain digit (the last digit which is the best estimate between the calibration marks). Titrate the KIO_3 solution. Pipet 25 mL of 0.5 M H_2SO_4. Begin titrating the KIO_3 solution immediate Experimental Procedure, Part A What is the procedure for preparing 250 mL of 0.0210 M Na_2S_2O_3 for this experiment from a 100 mL volume of standard 0.106 M Na_2S_2O_3?

Explanation / Answer

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?

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