Chemistry Lab Help!! Urgent!!!

Question

adamconner · Answer

John does not pay attention to the pre-lab discussions and has collected data incorrectly for the copper amine lab. No one realizes this until they begin working on the Midterm Module report, though. a. Review the following errors and “correct” them by providing a procedure or solution that would have fixed the issue. b. Decide whether the cases could be fixed without having to start the entire procedure over. If not, clearly state what would be a good point to restart.  c. Comment on how the error would affect the group’s accuracy and precision in each of the following cases (i.e., would it make the group’s average larger or smaller than it should be?). (Note that there are three items you must include for each of the scenarios.)

a.	When making dilutions for the calibration curve, John did not record an initial and final volume for the pipet but instead drained it completely. (2 pts)

b.	John made four dilutions as instructed, but the last two dilutions had a little Cu(OH)2 present. (2 pts)

adamconner · Answer

@aaronq can u help m with this one man?

homeworksucks · Answer

Classic John

homeworksucks · Answer

Um, for a it would make the value bigger than it should be

aaronq · Answer

a. a) not draining the pipet completely? lol  (this is one is kinda obvious)

b) if the addition of the remainder in the pipet was constant, then the calibration curve should shift all together. I'm guessing you were doing Absorbance vs concentration

|dw:1382826506862:dw|

if you know the extra volume that was added each time, then you can shift the graph.

c) if you don't correct it, it would make it seem like you had more than you actually did. The precision would be fine, but the accuracy wouldn't.

aaronq · Answer

|dw:1382826816757:dw|

adamconner · Answer

ok thank you so much!! what about the second scenario?

aaronq · Answer

ah looking at the graph i drew, i reversed the lines, it should be:
|dw:1382830043508:dw|

for a) you would have to add more ammonia to form the complex ion $Cu(NH_3)_4^{2+}$ and effectively measure the right concentration... Though, by doing this, you would dilute the solutions.

b) So you have to dilute the rest too, or start again.

c) $Cu(OH)_2$ is very insoluble, thus you would have more $Cu^{2+}$ than you measured. both the accuracy and precision would be off. Do you know the difference between the two?