Quimica analitica

Analytical chemistry lab 1
Treatment of experimental data
Sara Munoz
Lab partner: Eric Ziegler
08/28/2012

This experiment is the first lab of this semester; it is useful because it helps us understand how to calibrate our instruments to obtain a more accurate and precise data which is always useful. Whenever we want to make a measurement ( for instance, measure the weight of a flaskplus its constituents) we want to make sure that our value is very close to the true one, otherwise this error, even if at first it seems insignificant, it can propagate from calculation to calculation or all throughout this experiment and at the end we get a result that’s really far from what we expected or from the true value

This lab consists of two parts:
_ the first part is thecalibration of a pipette and the second part is the calibration of a burette.

In the first part, we used three different flasks of 125 ml each and labelled them 1,2,3 respectively. We also used a pipette of 25 ml and three stoppers for each flask.

Procedure:
Each flask was carefully cleaned and dried before the experiment started. We used acetone to dry them.
First, we weighed each flaskindividually with its stopper on and empty on an electronic balance. We weighed each flask three times to get a more accurate measurement of the mass of each flask. It turns out that we got the same measurement three times for the same flask. We recorded the results in the table sheet under empty mass.

We got for the first flask on average, from the three measurements taken:
. m1= 128.406 grams.For the second flask, we got :
. m2=115.763 grams
and the third;
.m3=122.872 grams.

Then, using the 25 ml pipette, we filled these three flasks with 25ml (this is the reading from the pipette’s line) each and weighed them again to see the difference in weight which is due to the added wáter.

For the first flask we got:
.m1’=153.304 grams
for the second:
.m2’=140.681 grams
forthe third one:
.m3’=147.783 grams.

Now, if we take the difference between the mass of the flask with water added and empty, we get how much water we added.

.dm1=m1’-m1=153.304-128.406=24.898 grams
.dm2=m2’-m2=140.681-115.763=24.918 grams
.dm3=m3’-m3=147.783-122.872=24.911 grams

we also measured the temperatura of each flask with water added; and recorded 24°C for each one of them.Now, by looking at the table data of density of water vs. temperature, we get that, for a temperature of 24°C, the density of the water is: pwater=0.9973 gr/ml

now, knowing the mass of the water added for each flask and the density of the water; we can calculate the volume delivered and compare that to the 25ml that we were supposed to deliver with the pipette.

By using the formula: V=m/pwhere p=density of the water at 24°C=0.9973 gr/ml

V1=dm1/p=24.898/0.9973=24.965ml
V2=dm2/p=24.918/0.9973=24.985ml
V3=dm3/p=24.911/0.9973=24.978ml

The average of these three values is

aveVolume=(24.965+24.985+24.978)/3=24.976ml

the standard deviation is:
sigma^2=[(24.976-24.965)^2+(24.976-24.985)^2+(24.976-24.978)^2]/3
= 6.8666*10^-5

sigma=0.0103577

the probable error is:P=0.6745*sigma
=0.006975

then, we corrected all the masses to vacuum.
For that, we used the formula in the lab book

Wvac= Wair[1+(pair/psub)-(pair/7.78)]

Since our temperature was 24°C, by looking at the table in p.5 we found that: pair(0.001188 g/ml)
The density of water is pwater=0.9973g/ml ( our substance is water)

Using all this information we found that:

For flask #1:Wvac1=Wair1[1+(0.001188/0.9973)-(0.001188/7.78)]
Where wair1=dm1
Wvac1=24.92365 grams very close to 25 ml which what we were supposed to add technically.
(all these results can be found in the excel sheet at the end of the lab report.)

Using the same procedure for the other flasks, we get:

For flask #2:
Wvac2=24.918[1+(0.001188/0.9973)-(0.001188/7.78)]
=24.98546 grams

flask #3:...