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April 30, 2016

# Homework Help: chemistry

Posted by davis on Saturday, March 24, 2007 at 10:21pm.

How many grams of NaCl should be added to 250 grams of water to make a 0.40 m solution?
I got 19 grams. this is how I got it.

first I found the mass of water = 18g
then I converted that to mols 18/250 = .0072.
I subtracted 0.072 - 0.40 = 0.328
I found the mass of NaCl = 58
0.328 x 58 = 19.024
that was my weird way of trying it. It may be wrong but I dont know any other way, thanks for your time

You want 0.4 m which, by definition, is
0.4 mol/kg water.
1 mol NaCl is 58.5 (you should look it up to get the exact number---58 isn't right--58.5 is closer but not exact). So for 1 kg of water you would want 0.4 x 58.5 = ?? grams.
For 250 g water, you would want 250/1000 = 1/4 of that amount.

0.4 x 58.5 = 23.4 grams is this my answer?

No. That is the amount in 1 kg of water. You only want 250 g (1/4 kg) so
23.4 x 250/1000 = ?? g/250 g. Then just to check yourself, remember the definition is mols/kg water. So take the grams, divide by 58.5 to convert to mols, then divide by 0.25 kg water and see if it is 0.4. If you have the right number for ?? g/250, it will be 0.4.

23.4 x 250/1000 = 5.85/250 = .0234
something's not right. I seriously dont know whats going on because I did all the calculations

Go back and read what I wrote.
0.4 m is 0.4mol/kg water.
0.4 mol is 58.5 g x 0.4 = 23.4 grams per kg water. But you have only 250 g of water or 1/4 of that amount so you need only 1/4 (250g/1000g) of the 23.4 g. Therefore,
23.4 x 250g/1000g = 5.85 g/250 and that's the answer. You want to add 5.85 grams NaCl to 250 g water to make a 0.4 m solution. THEN, I said, if we want to check it, just to see if we are right, we need to remember the definition. molality is mols/kg water.
So we put 5.85 g NaCl in 250 g water. What is the molality?
5.85/58.5 = 0.1 mol and 0.1/0.250 kg = 0.4 molal. So it checks. There is nothing to be confused about if you just remember the definition of molality. m = mols solute/kg solvent

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