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emunderpants · Mar 22, 2009, 04:34 PM

Calculate how many grams of NaOH there are in 500cm3 (cubed) of a 0.2 mol dm-3 solution?

Thanks a lot!

Perito · Mar 22, 2009, 04:37 PM

You know the concentration of NaOH. The concentration is in moles/deciliter. You also know the volume in cm3. Convert that volume to deciliters.

If you multiply X times Y/X, you'll end up with "Y". Right?
In the same vein, if you multiply moles/deciliter by deciliters, you'll end up with moles. That's what you're going to do. Once you know the number of moles, you can calculate the weight.

How? If you multiply grams/mole by moles you'll get grams. So, how many grams of NaOH are there in one mole of NaOH?

emunderpants · Mar 22, 2009, 04:59 PM

Thanks Perito but I'm still not sure of how to do this. If you could give an example of how to work it out it'd be a big help because I have a few more of these to do. Thanks a lot for your time.

Perito · Mar 22, 2009, 07:23 PM

1) Calculate molecular weight of NaOH:

Atomic weight of Na = 22.9898 grams/mole
Atomic weight of O = 15.9994 grams/mole
Atomic weight of H = 1.0079 grams/mole
Molecular weight of NaOH = 22.9898 + 15.9994 + 1.0079 = 39.9971 grams/mole

2) Calculate the number of moles of NaOH in 500 mL of 0.2 Moles/dL NaOH

Concentration of NaOH in moles/Liter:
0.2 M/dL NaOH * 10 dL/L = 2 Moles/Liter

Number of Liters in 500 $cm^3$ :
500 $cm^3$ * 1 L/ 1000 $cm^3$ = 0.5 L

Calculate the number of moles:
2 Moles/Liter * 0.5 Liter = 1.0 moles of NaOH

3) Knowing the number of moles, calculate the weight from the molecular weight:

1.0 moles NaOH * 39.9931 grams/mole = 39.9931 grams of NaOH.