Did a lab where I had to make known concentrations of ethanol and put them in ac

Question

Did a lab where I had to make known concentrations of ethanol and put them in acidic, basic and neutral conditions in a conway cell, otherwise known as micro-diffusion . In order to make the standards I had to weigh out exactly 400mg of ethanol and quantitatively transfer it into a volumetric flask. The question I need to answer is, is it necessary to weigh out exactly 400 mg of ethanol? why or why not? I am thinking that since they are asking me this question that the answer is no you dont because if you serial dilute it then you would account for the error of uncertaintanty but I am not 100 percent sure this is correct. If someone could explain that would be great!

Explanation / Answer

Ethanol usually contains 95% of alcohol and 5% water. So, in order to prepare known concentrations of ethanol having 400mg of ethanol, the actual amount required to be weighed out should be higher than 400mg, as 400mg ethanol would ideally contain 380mg of ethanol. So to prepare the above required solution larger amount (greater than 400mg) of ethanol needs to be added.

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