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.
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.
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.
When weighing liquid ethanol, you place water in the beaker prior to adding ethanol for two main reasons:
1. To create a stable environment: Ethanol has a tendency to evaporate quickly due to its low boiling point. By placing water in the beaker, you create a stable environment that helps slow down the evaporation of ethanol during the weighing process. This ensures a more accurate measurement of the ethanol.
2. To minimize errors due to temperature fluctuations: When you weigh a liquid substance, the weight can be affected by the temperature of the substance. By adding water to the beaker, you create a thermal buffer, which helps stabilize the temperature of the liquid and reduces the error caused by temperature fluctuations. This allows for a more precise measurement of the ethanol's weight.
In the specific case you mentioned, the purpose of adding 15 mL of water to the tared beaker is to provide a stable and consistent environment for weighing the 400 mg of absolute ethanol. The use of Parafilm to cover most of the opening further helps to reduce evaporation and maintain the stability of the liquid ethanol. This ensures that your measurements are accurate and reproducible, which is crucial when preparing a stock solution with a specific concentration like 0.40% (wt/vol).