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hike hike
wrote...
Posts: 9
Rep: 1 0
10 years ago
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
Source  general chemistry laboratory manual 9th edition by Beran
Read 11606 times
17 Replies

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Replies
wrote...
10 years ago
1.  Anywhere from 14.6 to 5.95 mg/l based on the temperature.
2. A) higher temp=less oxygen dissolved
B) higher pressure=less dissolved
C) not sure, is the volume of the water changing too?  My best guess is that bigger flask=more dissolved because there's more air above the water
D) more organic matter=less oxygen.  Not sure how it changes how O2 is dissolved but it can consume O2.
E) the deeper you go the less oxygen can be dissolved

Alright, so that's what, 2/5 of the questions? I'm sure someone else can help with the others Smiling Face with Open Mouth
Post Merge: 10 years ago

For #1 I have a source, I'm just not allowed to post links here :/
Just google "oxygen dissolved in water" and there should be a page from the EPA that has lots of good information Slight Smile
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hike Author
wrote...
10 years ago
Thank you so much for your help Slight Smile
wrote...
9 years ago
Is there any more answers to the rest of the questions? I really need them in case you have them Frowning Face Crying Face Crying Face Crying Face
wrote...
Valued Member
Educator
9 years ago
Is there any more answers to the rest of the questions? I really need them in case you have them Frowning Face Crying Face Crying Face Crying Face

Which one in particular?
wrote...
9 years ago
Starting from Question 3: a solution of MnSO4 till the last question of the whole Frowning Face
wrote...
9 years ago
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.
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Diesel
wrote...
Valued Member
Educator
9 years ago
Starting from Question 3: a solution of MnSO4 till the last question of the whole Frowning Face

Did goji.go's post help?
wrote...
9 years ago
Starting from Question 3: a solution of MnSO4 till the last question of the whole Frowning Face

Did goji.go's post help?

i still need answers to questions 4 and 5!
wrote...
9 years ago
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?

M1V1=M2V2

V2= 250x0.0210/0.106 = 49.52 ml

by taking 49.52 ml of 0.106M solution dilute it up to 250 ml by adding water so tha desired you will get desired one.

b)by using 25 ml pipet first take TWO TIMES 50 ml of 0.106M solution in volumetric flask then dilute solution up to the mark by adding water carefully.

concentration of resulting solution is

MOLARITY X VOLUME =0.0210X250/1000= 0.00525 M
wrote...
9 years ago
Thank you a lot .. It really helps!
wrote...
9 years ago
Thank you!
wrote...
3 years ago
...
wrote...
3 years ago
thank you
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