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.)
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.
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
and wouldn't the last one remain unchanged since it was at the end of the experiment and wouldn't affect it?
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.
1.) wouldn't it make the molarity less
2.) then that would make the vinegars molarity higher, right?
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.
To answer these questions, we need to understand the purpose of the lab procedures and their impact on the calculated molarity of the acid. Let's go through each question and explain how each procedure affects the final result:
1.) After rinsing the buret with distilled water, the buret is filled with the standard NaOH solution, and the acid is titrated to its equivalence point.
In this procedure, rinsing the buret with distilled water ensures that any impurities or residues are removed, preventing any contamination between different solutions. Filling the buret with the standard NaOH solution allows for accurate measurement and dispensing of the solution during titration. The molarity of the acid should not be affected by rinsing the buret with distilled water or filling it with standard NaOH solution since these steps do not introduce any additional substances that would react with the acid. Therefore, the calculated molarity of the acid would not be affected in this case.
2.) Phenolphthalein changes color around pH of 9 to signal the equivalence point, but an indicator that changes color around pH of 5 is used instead.
The choice of indicator affects the perception of the equivalence point during the titration. Phenolphthalein, which changes color around pH 9, is commonly used in acid-base titrations. If an indicator that changes color around pH 5 is used instead, it may give a false impression that the equivalence point is reached earlier than it actually is. This premature color change could lead to an inaccurate determination of the amount of NaOH required to reach the equivalence point, and consequently, the calculated molarity of the acid would be too high. Therefore, using an indicator that changes color around pH 5 would lead to a higher calculated molarity of the acid.
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).
If an air bubble is present in the buret during the titration, it can affect the accuracy of the volume measurements. The air bubble occupies a certain volume that is not accounted for in the volume calculations. As a result, less NaOH would be recorded as dispensed than what actually went into the acid solution. This leads to an underestimation of the amount of NaOH required to reach the equivalence point, resulting in a lower calculated molarity of the acid. Therefore, the presence of an air bubble in the buret would cause the calculated molarity of the acid to be too low.
In conclusion:
1.) Rinsing the buret with distilled water and filling it with the standard NaOH solution does not affect the calculated molarity of the acid.
2.) Using an indicator that changes color around pH 5 instead of pH 9 would result in a higher calculated molarity of the acid.
3.) The presence of an air bubble in the buret would cause the calculated molarity of the acid to be too low.