Question

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?

Answer 1

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. :-)