If 20 ml of water is added to a solution of ethanoic acid with a pH of 3, how will the solution change?

The new solution will be more diluted (less concentrated) and the pH will increase, i. e. the solution will become less acidic.

Explanation:

The pH of a solution is the logarithm of the inverse of the concentration (molarity) of the hydrodium ions ([H₃O⁺]):

pH = log 1 / [H₃O⁺]

When 20 ml of water is added to a solution of ethanoic acid with a pH of 3, the amount of hydronium ions is the same but the volume of solution is increased, meaning that the new concentration of hydronium ions is lower.

You can see that mathematically using the formula for molarity:

Molarity = number of moles of solute / volume of solution in liters

After adding water, the numerator of the formula remain unchanged while the denominator increases, so the new molarity is lower.

Now, since [H₃O⁺] is lower, its inverse, 1/[H₃O⁺] will be higher, and the logarithm, log 1/[H₃O⁺] will be higher.

Thus, you have concluded that the pH will be higher.

As per definition, a higher pH means less acidic (for instance, pH = 1 is highly acid and pH = 7 is neutral).

Then, the conclusion is that if 20 ml of water is added to a soluction of ethanoic acid with a pH of 3, the solution will be less concentrated, the pH will be higher, and the solution will be less acidic.