Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Fbala75 on April 17, 2005, 08:30:56 PM
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A stearic acid solution (molar mass 284 g/mol) is added drop wise into a water surface using a dropper pipet. It requires 17 drops of solution to form a monolayer on the water that ahs a surface area of 78.5 cm squared. Calculate the expirementle value of Avagadros number given the following data: Assume each stearic acid molecule ocupies an area of 20.5 A squared and that there are no spaces between molecules in the monolayer.
Data: Dropper pipet calibration is 90 drops. Stearic acid solution is 9.75 times ten to the negetive fifth g/ml 1 cm is 10 to the eighth A.
I have begun this problem by using the mass to mole and mass to mass ratios but i messed up and i do not know how to start it. Please would someone work out the problem for me includingshowing their work I would really apreciate it thanks a lot.
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I think you're missing a pipette droplet to volume ratio somewhere in there. Simply make efficient use of unit-factor cancellation to set up the problem.
- convert A2 to cm2 and divide the total surface area by the area covered by one molecule - this gives you the number of molecules
- calculate the volume of stearic acid dispensed onto the water's surface using the pipette droplet conversion factor - should end up with a volume in mL
- find grams using your conversion factor in g/mL
- find the number of moles
- Avogadro's number = # molecules/ mole = 6.02 * 1023
- divide the number of molecules by the number of moles - you should get a number on the order of magnitude of 1023