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 don't 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 don't 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
To calculate the number of grams of NaCl needed to make a 0.40 m solution in 250 grams of water, follow these steps:
1. First, determine the molar mass of NaCl. It is 58.5 grams/mol.
2. Next, calculate the amount of NaCl needed for 1 kg of water. Since the desired concentration is 0.4 m, multiply 0.4 m by the molar mass of NaCl (58.5 g/mol). This gives you 23.4 grams of NaCl per kg of water.
3. Now, since you have 250 grams of water, which is equivalent to 0.25 kg, you need to find 1/4th of the amount calculated in step 2. Multiply 23.4 grams by 0.25. This gives you 5.85 grams of NaCl.
Therefore, you need to add 5.85 grams of NaCl to 250 grams of water to make a 0.40 m solution.