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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Seymor-Omnis on March 24, 2004, 04:10:24 PM

Title: Percent Composition Question
Post by: Seymor-Omnis on March 24, 2004, 04:10:24 PM
Today we did a lab and the teacher made a mistake...quite a large one in fact  ::).

The lab required 20 ml of 20% NaOH.  First she read the lab wrong and thought it said 10%.  Second she just moved the decimal to create a .1 molar solution.

This was very wrong, and I am wondering why.  How do you calculate percent composition?  Any help is greatly appreciated.
Title: Re:Percent Composition Question
Post by: Mitch on March 24, 2004, 04:27:33 PM
There are a lot of ways to calculate it. She was probably doing it by volumes instead of by moles. When I make a 10%HCl solution, its usally 1 part 12M HCl with 9parts water.
Title: Re:Percent Composition Question
Post by: Seymor-Omnis on March 24, 2004, 06:15:05 PM
ok...that was very informative...except i did not understand any of it  ;D.

Could you explain it again?
Title: Re:Percent Composition Question
Post by: gregpawin on March 24, 2004, 08:10:41 PM
As Mitch mentioned there could be different ways of doing it... you could be talking about moles, mass, or volume.

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fgregpawin.chemicalforums.com%2Fpercentcomposition.gif&hash=ccb75cbe9d4840eac4d282b82561e7b1)

Because in the end all you get is a ratio, confusion is common.  That's why usually, if you're going to talk about concentration using percent composition, by "weight/mole/volume" should be mentioned.