Sunday
December 21, 2014

Homework Help: Chemistry (3rd time I've posted this)

Posted by Joe on Wednesday, March 21, 2007 at 9:09pm.

Can someone check my answers? I've posted this a couple times and no one has responded. Is no one responding because they're all right or what?

1.) If you were involved in an acid rain monitoring project with several other samplers, why would it be important that all of you collect your samples on the same day and do the alkalinity tests within 24 hours?

Acid rain on another day may not have the same pH as the rain on that day. The pH may also decrease over time.

2.) A student did an alkalinity determination correctly, except that the sample titrated had a volume of 90.0 mL. Volume A was found to be 4.50 mL and Volume B was 7.00 mL.
a)Calculate the alkalinity value from these data and then calculate the correct value.
A=(2A-B)
A=(2(4.50-7.00)=2.0 mL^-1
A=(2(4.545)-7.07)
answer = 2.02mL^-1

b)What percent error would there be?
2.02-2.0
--------
2.02
x100

answer = .99%

c) Would there be an error in the classification of the stream? Explain concisely.

There would not have been an error in the classification. It was just a measurement error in this case.

3) EPA acidity tests can be performed by titrating a water sample to a pH of 8.2. What titrant in what concentration would you suggest for EPA acidity determinations?

The titrant I would suggest for EPA acidity determinations is sulfuric acid under a standard concentration.

And I've read it all three times. Sone of the questions I simply dn't uuderstand. For what it's worth, here goes.
1. I think your answer to #1 is poor. Probably everyone knows that the pH may/will change. Probably they also know that the pH will change over time. I think the question is asking, "WHY will the pH be different on different days and WHY will the pH decrease over time. You might address that the same amount of rain may differ on diffrerent days, that the rain front may have come from different loations of the world thus carrying different amounts of gases that causes the rain to be acid, etc. As to the alkalinity changing, WHY does it change; that is, WHY is it important to measure the alkalinity within a short time of sample collection. The answer is that the dissolved gases causing the rain to be acid have a vapor pressure and that over time they will slowly "evaporate" from the solution. Evaporate is not a good word but you can substitute a better one. You probably knew all of this but you need to spell it out, especially if this is for an assignment. If it's for general dissemination, it is even more vital to be specific.

2. I don't understand the volume A and volume B bit so I've skipped all of #2.

3. Again, I don't think you answer will cut it but it is a better answer than your #1. You were specific about using H2SO4 and that is good. I think the answer would be greatly imporoved if you specified the recommended concentration or give a range. To say "a standard concentration" will make the reader (your prof?) glance at the paper again and he/she may wonder if you know or if you are just evading the question.

I hope this helps a little although I couldn't answer all of the question.
\

And my addition. I have read it three times also, and ibid on DrBob's comments. It is confusing.
On the last, my thought is IF you are titrating an acid rain water sample you need a standard base sample (NaOH), especially if you want it to get to a pH of 8. However, it is not clear to me in number three what you are trying to titrate.
The clarity of your questions makes understanding very difficult. If you ask someone something three times, and they dont answer, it is quite likely they dont understand what you are talking about. If those folks are experts, and dont understand you, well, think about that. It is part of learning the art of communication.

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