[Seymor-Omnis]

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.

[Mitch]

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.

[Seymor-Omnis]

ok...that was very informative...except i did not understand any of it  .

Could you explain it again?

[gregpawin]

As Mitch mentioned there could be different ways of doing it... you could be talking about moles, mass, or volume.



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.