If 400 mL of a 20% w/v solution were diluted to 2 L, what would be the final percentage strength?

4%

Step-by-step explanation:

A 20% solution with a total volume of 400 mL has 20% * 400 mL of solute.

20% * 400 mL = 80 mL

When you dilute the solution to 2 L, you introduce additional water, but no additional solute, so now you have the same 80 mL of solute in 2 L of total solution.

The concentration is:

(80 mL) / (2 L) = (80 mL) / (2000 mL) = 0.04

As a percent it is:

0.04 * 100% = 4%