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.
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.
2 answers
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.