Question

which description correctly characterizes the acidity or basicity of a solution?
○ the higher the ph is, the more the hydroxide ion concentration decreases and the more acidic the solution becomes
○ the higher the ph is, the more the hydroxide ion concentration increases and the more basic the solution becomes
○ the lower the ph is, the more the hydronium ion concentration decreases and the more acidic the solution becomes
○ the lower the ph is, the more the hydronium ion concentration increases and the more basic the solution becomes

Explanation:

1. Recall the relationship between pH, hydronium ion ($\ce{H3O+}$) and hydroxide ion ($\ce{OH-}$) concentrations:

• The pH scale is defined as $\text{pH}=-\log_{10}[\ce{H3O+}]$. So, as pH increases, $[\ce{H3O+}]$ decreases.
• In an aqueous solution, the ion - product constant of water $K_w = [\ce{H3O+}][\ce{OH-}]$ (at a given temperature, usually $25^{\circ}\text{C}$ where $K_w = 1.0\times10^{-14}$). If $[\ce{H3O+}]$ decreases, to keep $K_w$ constant, $[\ce{OH-}]$ must increase.
• A solution with a higher concentration of $\ce{OH-}$ (and lower concentration of $\ce{H3O+}$) is basic. A solution with a higher concentration of $\ce{H3O+}$ (and lower concentration of $\ce{OH-}$) is acidic.

1. Analyze each option:

• Option 1: If pH is higher, $[\ce{H3O+}]$ is lower, $[\ce{OH-}]$ should increase (not decrease), and the solution is more basic (not acidic). So this option is wrong.
• Option 2: As pH increases, $[\ce{H3O+}]$ decreases, $[\ce{OH-}]$ increases (from $K_w = [\ce{H3O+}][\ce{OH-}]$), and a higher $[\ce{OH-}]$ means a more basic solution. This option is correct.
• Option 3: If pH is lower, $[\ce{H3O+}]$ is higher (not lower) because $\text{pH}=-\log_{10}[\ce{H3O+}]$. A higher $[\ce{H3O+}]$ means a more acidic solution, but the statement about $[\ce{H3O+}]$ decreasing is wrong.
• Option 4: If pH is lower, $[\ce{H3O+}]$ is higher, and a higher $[\ce{H3O+}]$ means a more acidic (not basic) solution. So this option is wrong.

Answer:

B. The higher the pH is, the more the hydroxide ion concentration increases and the more basic the solution becomes.