Question

After performing a dilution calculation, you determine you need 25.0 milliliters of an aqueous stock solution to make 100.0 milliliters of a new solution. How would this be prepared?

Answer 1

A is obviously out because it leads to a volume of 125.0 milliliters of the new solution and gives you a lower concentration than you were aiming for.

D is out because you are adding 75 milliliters of the stock solution, so your concentration would be too high. You only need 25.0 milometers of stock solution per 100 milliliters of the new solution.

C is also out because it leads to 50.0 milliliters stock solution per 100 milliliters of the new solution and hence the wrong concentration.

B is by default the correct answer. It also details the correct technique. First you add the stock solution (This you know from your calculations to be 25 milliliters.) then you add the water up to the volume you needed. (Because the calculations only tell you the total volume of water not what you need to add) You also add the water last so you can rinse the neck of the flask to make sure you also get all the stock solution residue into the stock solution.

I would add the final step of stirring, but B is the only answer that can be correct.

Answer 2

Answer:

You add 75 ml of water to a volumetric flask, along with the 25 ml of stock solution, to prepare a 100 ml dilution

Explanation:

Dilution is the procedure used to prepare a less concentrated solution from a more concentrated solution. It consists of adding solvent to an existing solution. Then the amount of solute does not vary, but the volume of the solvent does: when more solvent is added, the concentration of the solute decreases, as the volume of the solution increases.

In the case of a dilution, a Dilution Factor can be determined, which is a number that indicates how many times a solution must be diluted to obtain a lower concentration, such as:

Dilution factor = $\frac{Vfinal}{Vinitial}$

where Vfinal is the volume of the stock solution and Vinitial is the volume of the diluted solution.

In this case you need 25.0 ml of a stock solution to make 100.0 ml of a new diluted solution. So the dilution factor is:

Dilution factor=$\frac{100}{25}$

Dilution factor=4

This means that the initial solution must be diluted 4 times. Another way to see this is that for every 4 parts of this new diluted solution, 1 part is represented by the stock solution and 3 parts by the diluent water. Then, to prepare this solution, you must add 25.0 ml of stock solution to a volumetric flask, then add water until the total volume is equal to 100.0 ml.

So, you can do the calculation:

100 ml - 25 ml = 75 ml

This means that you add 75 ml of water to a volumetric flask, along with the 25 ml of stock solution, to prepare a 100 ml dilution.  You diluted the stock solution by a factor of  4.