To make a 4% paraformaldehyde solution, starting from a 20% solution diluted with PBS, you can use the formula V1C1=V2C2, where V1 is the volume of the starting solution (20%), C1 is the concentration of the starting solution (20%), V2 is the desired final volume, and C2 is the desired final concentration (4%).
In this case, you want to calculate the volume of the starting solution (20%) to mix with PBS in order to obtain a 4% paraformaldehyde solution.
By plugging in the values into the formula, you have (20%)(100) = (V2)(4%).
To solve for V2, you divide both sides of the equation by 4%, giving you V2 = (20%)(100) / 4%.
Calculating this, you get V2 = 500 ml.
Therefore, you need to add 500 ml of PBS to the starting 20% solution to obtain a 4% paraformaldehyde solution.