When weighing liquid ethanol, why do you have to place water in the beaker prior to adding the ethanol to that same beaker?
I have no idea but I'd like to know when you find out. If you want to know the mass of the ethanol you won't get it this way. Perhaps if you explained how this question was generated (what experiment/subject/procedure) I may be able to get a better handle on it.
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.
When weighing liquid ethanol, water is placed in the beaker prior to adding the ethanol in order to prevent evaporation or loss of ethanol during the weighing process. The reason behind this is related to the concept of hygroscopicity.
Ethanol is hygroscopic, meaning it has the ability to absorb or attract moisture from the surrounding environment. If you directly weigh ethanol in an open beaker, it can absorb moisture from the air, leading to a change in its weight. This change in weight would not accurately reflect the actual weight of ethanol present.
To prevent this, water is added to the beaker before adding ethanol. The water creates a humid environment inside the beaker, saturating the air with moisture. This minimizes the interaction of ethanol with the surrounding air and prevents it from absorbing additional moisture.
By allowing the beaker to reach a state of equilibrium with humidity, the added water provides a stable environment for weighing the ethanol accurately. The weight measured after adding ethanol will then mostly reflect the true weight of ethanol itself, rather than a combination of ethanol and absorbed atmospheric moisture.
To obtain an accurate measurement, it is important to tare or zero the balance before adding the beaker with water and ethanol. This ensures that only the weight of the liquid in the beaker is measured, not the weight of the beaker itself.