Dilution: Moles to Volume

Question

You wish to make a 0.240 M hydrochloric acid solution from a stock solution of 12.0 M hydrochloric acid. How much concentrated acid must you add to obtain a total volume of 175 mL of the dilute solution?

 ________mL

Answer 1

You can use V1M1 = V2M2 for this type of problem.

V1 = initial volume = x ml

M1 = initial concentration = 12.0 M

V2 = final volume = 175 ml

M2 = final concentration = 0.240 M

(x ml)(12.0 M) = (175 ml)(0.240 M)

x = 3.5 ml

So, you would have to add 3.5 ml of 12.0 M and then dilute this to a final volume of 175 ml to get a 0.240 M solution. NOTE: you don't add this amount of 12 M acid to the 175 ml, but rather the FINAL VOLUME will be 175 ml. The question is poorly worded.

Answer 2

Use M₁V₁ = M₂V₂ for dilution of solution problems. The idea is the number of mole of the solute does not change upon dilution.

You only group M and V for solutions 1 and 2 and make sure both volumes are in the same unit mL (or L).