Tuesday
March 31, 2015

Homework Help: AP Chemistry

Posted by Ellan on Thursday, October 1, 2009 at 8:26pm.

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.)

My thoughts:

Wouldn't number 2 create a higher molarity since the lower pH would make it acidic?

and number 3 wouldn't it stay relatively unchanged since the bubble was formed after the experiment?

Thanks for your help :]

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