you have 505mL of a .125 M HCl solution and you want to dilute it to exactly .100. How much water should you add?

I got (.125M)(505mL)=(.100M)(Vf)=
631.25. But on my answer sheet they got 126 mL of H2O???? I don't know how she got it.

The volume you calculated, 631.25 mL, is the total volume. The question was how much water should be added and that is

631-505 = 126 mL.
(I should point out something. Technically total volume should be 631.25 AND 126 added to 505 will not give EXACTLY 631 mL. because volumes are not EXACTLY additive. Probably I would not have mentioned this if the word exactly had not been present). The point I'm making is that solutions should be made to a specified volume and that is usually done with a volumetric flask. Adding a certain volume of water is not proper and I am not familiar with flasks that can be made to an exact volume of 631 (or 631.25).

To dilute the HCl solution to a desired concentration, you need to use the formula for dilution:

C1V1 = C2V2

where:
C1 = initial concentration of the solution
V1 = initial volume of the solution
C2 = final concentration of the solution
V2 = final volume of the solution

In this case, we have:
C1 = 0.125 M
V1 = 505 mL
C2 = 0.100 M

Let's solve for V2, the final volume of the solution:

0.125 M * 505 mL = 0.100 M * V2

(0.125 M * 505 mL) / 0.100 M = V2

V2 = (0.125 * 505) / 0.100
V2 = 631.25 mL

So, the correct answer is 631.25 mL, not 126 mL.

It seems there might be an error on your answer sheet. Please verify this with your instructor or refer to the correct solution.

To determine the amount of water you need to add to the .125 M HCl solution to dilute it to exactly .100 M, you can use the concept of dilution. Dilution involves reducing the concentration of a solution by adding more solvent (in this case, water).

The formula you used, C1V1 = C2V2, is the correct formula to use for dilution calculations, where C1 and V1 represent the initial concentration and volume of the solution, and C2 and V2 represent the final concentration and volume of the solution.

Let's break down the equation and solve it step by step:

C1V1 = C2V2

Given:
C1 = 0.125 M (initial concentration)
V1 = 505 mL (initial volume)
C2 = 0.100 M (final concentration)

We want to find V2, which represents the volume of the final solution.

Substituting the given values into the equation:

(0.125 M)(505 mL) = (0.100 M)(V2)

Now, let's solve for V2:

(0.125 M)(505 mL) = (0.100 M)(V2)
63.125 = 0.100 V2

Divide both sides by 0.100:

63.125 / 0.100 = V2
V2 = 631.25 mL

So, according to the calculations, the final volume should be 631.25 mL.

It seems like there may have been a mistake on your answer sheet. The correct answer is 631.25 mL, not 126 mL.