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### Topic: Help with Avagadros Number Problem  (Read 5524 times)

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#### Fbala75

• Guest ##### Help with Avagadros Number Problem
« on: April 17, 2005, 08:30:56 PM »

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.

#### Vette Freak

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• Gender: • Go Terps! ##### Re:Help with Avagadros Number Problem
« Reply #1 on: April 18, 2005, 09:40:55 AM »
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
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