[CHM 111] Lab Report question

[Andrew J. Pounds]

On 9/5/20 12:33 PM,  wrote:
> Dear Dr. Pounds, 
>
> I have been working on the lab report, and I am a tad confused. There
> is a chart for exact volume and the masses from the lab, but it says
> that those numbers should not be what we measured. I am a little
> confused as to what they are and how I am to find them from the
> measured mass and such, as I thought what we measured is what we were
> supposed to use to find density. However, it says that none of those
> values should be what we found in table 1 of the lab. Why would we not
> use the masses and volumes that we measured in the lab?
>
> Thank you for your time,
>
>

So let's see if I can clarify this -- because I didn't write those
procedures.  In the lab you recorded the initial volume in the buret. 
Let's imaging that was 0.38 ml.  The procedure then tells you to place
approximately 1.00 ml of water in the beaker.  That means your next
buret reading should have been close to 1.38 -- but after watching
students for years, I can bet that it wasn't exactly 1.38.  For the sake
of this example, we'll say that it was 1.41.  


Similarly, your initial beaker mass might have been something like 55.33
grams on the to loading balance and 55.3632 on the analytical balance.  
After your added approximately 1.00 ml it might have been something like
56.38 grams on the top loading and 56.4231 grams on the analytical balance


Those are all values that you measured.  The actual volume of water for
the "~1 ml" case would be 1.41-0.38 = 1.03 ml.   The mass of the drop of
water from the top loading balance would be  56.38 - 55.33 = 1.05 grams
and from the analytical balance it would be 56.4231 - 55.3632 = 1.0599
grams.


The density of water from the top loading balance would then be 1.05 g /
1.03 ml = 1.02 g/ml


The density of water from the analytical balance would be 1.0599 g /
1.03 ml = 1.03 g/ml


So the first line of the table on the report form would be have, using
my fictional data, the entries, 1.03 ml,  1.05 g,  1.02 g/ml,  1.0599 g,
and  1.03 g/ml respectively.


Note -- none of those values were found in your original Table 1 of the
lab procedures because they were all calculated from those data.  Does
that help?




-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Director of the Computational Science Program
Mercer University,  Macon, GA 31207   (478) 301-5627

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