Question

I need help with these last two questions, thanks!

Answer 1

Both of these questions can be solved using the equation M1V1 = M2V2, where M is concentration anf V is volume.

For the first case, M2 = 0.2 mol/L, M1 = 3 mol/L, and V2 = 250mL. So now you want V1. Solving for V1, V1 = (M2 / M1)V2 =
(0.2 / 3)(250) = 16.7 mL. So what that means is that you need 16.7 mL of 3M HCl, and the rest of the 250 mL (which would be 250 - 16.7 = 233.3 mL) would be water, with which you're diluting the HCl.

Same principle for the second problem, except now we have percentages and not mol/L. You can treat the percentages as concentrations. Since you're starting with pure isopropyl alcohol, M1 = 100%. You want a final volume of 500 mL and a final concentration of 70%. To find the volume of isopropyl alcohol you need to start with, solve for V1. So V1 = (M2 / M1)V2 = (70 / 100)(500) = 350 mL. So you need 350 mL of isopropyl alcohol and the rest of the 500 mL (that is, 150 mL) you can fill with water.