Lab Manual Update

I spent some time in lab today to do a couple of things:

1) I tested the change in volume when mixing water and NaCl.  I found out two things:

a) After adding 5 g of NaCl (in stages) to exactly 100 mL of water in a volumetric flask, there was about a 2-3 mL increase in volume, contrary to what I have always thought.

b) I added 5 g of NaCl to an empty 100 mL volumetric flask and then added enough water to the solid up to the 100-mL line.  After mixing, the volume went down.  I believe this is what I observed when students in the 30A brought me their solution with the liquid surface below the line.

I did not test glucose and water as I did not want to waste glucose but the physio text is correct:  Adding 5 g of glucose to 100 mL of water will not make a 5% solution because the volume will be more than 100 mL.

2) I made up a 1% glucose solution but made an error making up a 0.1% solution (I did a 100 fold dilution instead of 10-fold dilution).  I did not realize this until later when I measured no absorbance (it was a 0.01% solution!  Too dilute).

I was able to measure an absorbance at around 490 - 500 nm for the 10 mL 1% glucose + 20 drops of DNSA.  The color change to reddish, burgundy hue was actually quite visible.  Still, this is not close to 525 nm or 575 nm from the original lab.

Tomorrow, I will try 3 mL of DNSA and 3 mL of 0.1%, following exactly what the other lab did.

I will also set-up standards using the 1.0% glucose as stock and measure the absorbance.  The absorbance of the 1.0% + 20 dropos DNSA was only 0.7 at the most.  I wonder if I should add more DNSA?  I will test this tomorrow.