Which is more concentrated, 500mL of a .20M solution of NaCl or 250mL of a .25M solution of NaCl?
MMNaCl = 58.44247
M x V = n
.5L x .2M = .1moleNaCl
.1moleNaCl x (58.44247gNaCl / 1moleNaCl) = 6gNaCl
.25L x .25M = .0625moleNaCl
.0625moleNaCl x (58.44247gNaCl / 1moleNaCl) = 3.653gNaCl
I think the first one is more concentrated from my calculations but I know the answer says it's supposed to be the second. What did I do wrong?
You got sucked in. The question did NOT ask for grams NaCl; it asked for the concentration of NaCl and the problem TOLD you #1 was 0.2M and #2 was 0.25M. 0.25M is a larger number than 0.2. All of the work you did is for naught. :-)
Based on your calculations, it seems like you have made a slight mistake in the conversion from moles to grams. Let's go through the calculations again:
First, let's calculate the amount of NaCl in the 500 mL of the 0.20 M solution:
Volume of solution = 500 mL = 0.5 L
Concentration of solution = 0.20 M
Moles of NaCl = Concentration x Volume
Moles of NaCl = 0.20 M x 0.5 L = 0.1 moles
Now, let's convert moles to grams using the molar mass of NaCl:
Molar mass of NaCl = 58.44247 g/mol
Mass of NaCl = Moles x Molar mass
Mass of NaCl = 0.1 moles x 58.44247 g/mol = 5.844247 g
Therefore, the mass of NaCl in 500 mL of the 0.20 M solution is 5.844247 grams.
Now, let's calculate the amount of NaCl in the 250 mL of the 0.25 M solution:
Volume of solution = 250 mL = 0.25 L
Concentration of solution = 0.25 M
Moles of NaCl = Concentration x Volume
Moles of NaCl = 0.25 M x 0.25 L = 0.0625 moles
Now, let's convert moles to grams using the molar mass of NaCl:
Molar mass of NaCl = 58.44247 g/mol
Mass of NaCl = Moles x Molar mass
Mass of NaCl = 0.0625 moles x 58.44247 g/mol = 3.652654 g
Therefore, the mass of NaCl in 250 mL of the 0.25 M solution is 3.652654 grams.
After recalculating, we can see that the mass of NaCl in the 500 mL of the 0.20 M solution is indeed higher at 5.844247 grams compared to the 250 mL of the 0.25 M solution with 3.652654 grams. Therefore, your initial calculation was correct, and the first solution is more concentrated than the second solution.