#### Answer

(b) The acidity decreases.

#### Work Step by Step

As we can see in figure 16.8, the pH represents more acidic solutions with small values, and basic solutions with values greater than 7.0.
The acidity of a solution is proportional to its $H_3O^+$ concentration.
The mathematical definition of pH is:
$$pH = -log[H_3O^+]$$
Since there is a negative sign in the logarithimic expression, $[H_3O^+]$ is inversely proportional to the pH.
Therefore, the pH is inversely proportional to $[H_3O^+]$, which is proportional to the acidity, making the pH inversely proportional to the acidity.