Homework Help: chemistry

Posted by Jenny on Thursday, October 1, 2009 at 7:48pm.

So i had trouble finding the answers to these questions on a lab:

Background: If we were using a burret/flask instead of a pipette and well plates. Before you started, the buret must be rinsed with 1 ml of distilled water and then 1 ml of NaOH solution. The flask would also be rinsed with 1 ml of distilled water and 1 ml of vinegar.

Questions: Based on the background info. above, predict how the calculated molarity of the acid would be affected (too high, too low, or not affected) by the following lab procedures. (then explain your answer)

1.) After rinsing the buret with distilled water, the buret is filled with the standard NaOH solution; the acid is titrated to its equivalence point.

2.) Phenolpthalein changes color around pH of 9 to signal the equivalence point; an indicator that changes color around pH of 5 is used instead.

3.) An air bubble of some size was present in the buret at the end of the experiment (it was pushed in after the lab started.)

chemistry - DrBob222, Thursday, October 1, 2009 at 8:13pm
Do you have any idea what these answers are? Or at least the approach you might take? Just giving away three answers is not my style.
chemistry - Jenny, Thursday, October 1, 2009 at 8:15pm
I know, and i'm sorry. I am having a lot of trouble with these ones.

I thought for the second one. it would cause a higher molarity since the pH was lower, but i'm still not sure
chemistry - Jenny, Thursday, October 1, 2009 at 8:17pm
and wouldn't the last one remain unchanged since it was at the end of the experiment and wouldn't affect it?
chemistry - DrBob222, Thursday, October 1, 2009 at 8:25pm
1). Adding NaOH to the buret after rinsing the buret with distilled water (essentially saying that you don't rinse the buret with NaOH)means that the NaOH is more dilute so it will take more NaOH from the buret to reach the equivalence point. So more mL means mL x M = a greater number which will do ?????? for the M of the vinegar?

b)Using an indicator that changes around 5 instead of about 9 will mean you must add more NaOH to get the pH to move from 5 to 9. More mL NaOH, means more M x mL = more moles NaOH and that will do what to the M of the vinegar?

c) I don't know how to answer this one because I don't understand the question. Does pushing the bubble in after the lab started mean extra NaOH was pushed out? Or does it mean something different? I don't know.
chemistry - Ellan, Thursday, October 1, 2009 at 8:41pm
1.) wouldn't it make the molarity less

2.) then that would make the vinegars molarity higher, right?
chemistry - DrBob222, Thursday, October 1, 2009 at 10:53pm
1.) wouldn't it make the molarity less
I don't see it that way.
mLNaOH x M NaOH = mLv x Mv so
Mv = mLNaOH x M NaOH/mLv.
Increasing mL NaOH increases the numerator, mL v in the denominator stays the same so the M of the vinegar must increase.

2.) then that would make the vinegars molarity higher, right?
Same answer for the same reason.

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