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

Here's what I got.

Explanation:

You know that you need `"25.0 mL"` of a stock solution to make `"100.0 mL"` of a new, diluted solution. This basically means that you're going to dilute your stock solution by a factor of

`(100.0color(red)(cancel(color(black)("mL"))))/(25.0color(red)(cancel(color(black)("mL")))) = 4`

You thus know that for every `4` parts of this new, diluted solution, `1` part is accounted for by the stock solution and `3` parts are accounted for by water, you diluent.

The trick now is to realize that despite the fact that you'd need

`"100.0 mL " - " 25.0 mL" = "75.0 mL"`

of water to make this new solution, you should not add exactly `"75.0 mL"` of water to your `"25.0 mL"` of stock solution. Instead, you should add enough water to make sure that the total volume of the solution is equal to `"100.0 mL"`.

So, to prepare this solution, you should add `"25.0 mL"` of stock solution to a volumetric flask, then add water until the total volume is equal to `"100.0 mL"`. You just diluted your stock solution by a factor of `4`.