Question

When weighing liquid ethanol, why do you have to place water in the beaker prior to adding the ethanol to that same beaker? We were told to make a to make a 0.40% (wt/vol) stock solution by weighing 400 mg of absolute ethanol into a tared beaker that contained 15 mL of water and has most of the opening covered with Parafilm.

Answer 1

I think the answer is mostly one of reducing the evaporation of the ethanol. That is especially true if the ethanol is being used as a standard for some measurement.
The vapor pressure of absolute ethanol at 19 C is something of the order of 40 mm Hg. If you add the water first, then add the ethanol, the concn of ethanol (0.4g/15 mL) is <1% and the vapor pressure is about 5 mm or so. The covering reduces the evaporation of the solution, also. If you are making this as a standard, I'm unsure how you're making the volume up to 100 mL solution (unless, of course, you are pouring the ethanol/H2O mixture into a volumetric flask) and making to the mark.

Answer 2

Thank you so much ! I was looking everywhere and couldn't figure out an answer this makes sense though. And yes the mixture was transferred to a volumetric flask after the ethanol was weighed out.