pH of Strong Acids and Bases: Videos & Practice Problems

pOH Calculation Example

To calculate the pOH of a barium hydride solution with a concentration of 0.398 molar, we first recognize that barium hydride (BaH₂) is a strong base that completely dissociates in solution. The dissociation can be represented as:

BaH₂ → Ba²⁺ + 2 H^-

From this reaction, we see that one mole of barium hydride produces two moles of hydride ions (H^-). Therefore, the concentration of hydride ions is:

[H^-] = 2 × 0.398 mol/L = 0.796 mol/L

Since hydride ions (H^-) can be considered equivalent to hydroxide ions (OH^-) in terms of their effect on pOH, we can use this concentration to find the pOH of the solution. The formula for calculating pOH is:

pOH = -log[OH^-]

Substituting the concentration of hydroxide ions into the equation gives us:

pOH = -log(0.796)

Calculating this yields a pOH of approximately 0.0991. Thus, the pOH of the barium hydride solution is 0.0991, indicating a highly basic solution.

An aqueous solution of HBrO₄ has a pH of 4.34. Find the molar concentration of HBrO₄ solution.

• A. 4.6 × 10⁻¹⁹ M
• B. 2.6 × 10⁻⁶ M
• C. 2.2 × 10⁻¹⁰ M
• D. 4.6 × 10⁻⁵ M

Answer: (D)

HI is a strong acid (K_a = 3.2 × 10⁹). Calculate [H⁺], [OH⁻], pH and pOH of a 7.1 × 10⁻² M HI solution.

• A. [H⁺] = 7.1 × 10⁻² M
  [OH⁻] = 1.4 × 10⁻¹³ M
  pH = 1.15
  pOH = 12.85
• B. [H⁺] = 7.1 × 10⁻² M
  [OH⁻] = 1.4 × 10⁻¹³ M
  pH = 8.50
  pOH = 5.50
• C. [H⁺] = 7.1 × 10⁻² M
  [OH⁻] = 7.1 × 10⁻¹⁶ M
  pH = 1.15
  pOH = 15.15
• D. [H⁺] = 7.1 × 10⁻² M
  [OH⁻] = 7.1 × 10⁻¹² M
  pH = 1.15
  pOH = 11.15

Answer: (A)

Here's what students ask on this topic:

What is the relationship between the concentration of a strong acid and the concentration of hydrogen ions (H⁺) in solution?

Strong acids completely ionize in water, meaning they dissociate 100% into their ions. For example, if you have a 0.25 M solution of hydrochloric acid (HCl), it fully dissociates into H⁺ and Cl^- ions. This means the concentration of H⁺ ions is equal to the initial concentration of the acid, which is 0.25 M. This direct relationship simplifies pH calculations because the concentration of H⁺ ions can be taken as the molarity of the strong acid.

How do you calculate the hydroxide ion (OH^-) concentration from a strong base like calcium hydroxide?

Calcium hydroxide (Ca(OH)₂) dissociates completely in water, producing one Ca²⁺ ion and two OH^- ions per formula unit. If the molarity of Ca(OH)₂ is 1.2 M, the concentration of OH^- ions is twice that, because each molecule produces two hydroxide ions. Therefore, the OH^- concentration is calculated as $C_{\mathrm{OH}}=\; 2\; imes\; 1.2\; =\; 2.4\; ext\{\; M\}$. This doubling effect is important when calculating pOH and pH for strong bases that release multiple hydroxide ions.

What is the formula that relates pH and pOH, and how is it used in acid-base calculations?

The relationship between pH and pOH is given by the equation $\mathrm{pH}+\mathrm{pOH}=\; 14$. This means if you know either the pH or pOH of a solution, you can find the other by subtracting from 14. This is useful because pH measures the acidity (H⁺ concentration) and pOH measures the basicity (OH^- concentration). For example, if a solution has a pH of 3, then its pOH is 11. This relationship helps in understanding the balance of ions in aqueous solutions.

How do you calculate pH from the concentration of hydrogen ions (H⁺)?

pH is calculated using the formula $\mathrm{pH}=\; -ext\{log\}\_\{10\}\; [ext\{H\}^+]$, where [H⁺] is the molar concentration of hydrogen ions. For example, if the H⁺ concentration is 0.25 M, then $\mathrm{pH}=\; -ext\{log\}\_\{10\}\; (0.25)\; \backslash \backslash \; \backslash ,\; \backslash (\backslash approx\backslash )\; 0.60$. This logarithmic scale means that small changes in H⁺ concentration correspond to significant changes in pH, which is why pH is a convenient way to express acidity.

What ions are considered equivalent to hydroxide ions (OH^-) when calculating pOH in strong bases?

In addition to hydroxide ions (OH^-), strong bases may produce other basic anions such as hydride (H^-), amide (NH₂^-), and oxide (O^2-) ions. When calculating pOH or the basicity of a solution, the concentration of these ions is treated as equivalent to the concentration of OH^- ions. This means their presence contributes to the overall basicity and must be included in calculations involving pOH and pH.

How do you find the hydroxide ion concentration from pOH?

The hydroxide ion concentration can be found from pOH using the formula $[ext\{OH\}^-]\; =\; 10^\{-ext\{pOH\}\}$. For example, if the pOH of a solution is 3, then the OH^- concentration is $10^\{-3\}\; =\; 0.001\; ext\{\; M\}$. This inverse logarithmic relationship allows you to convert between pOH and hydroxide ion concentration easily, which is essential for understanding the basicity of a solution.