pH of Strong Acids and Bases - Online Tutor, Practice Problems & Exam Prep

Strong acids and bases fully ionize in water, meaning their concentrations equal the concentrations of H⁺ and OH^- ions, respectively. For example, a 0.25 M hydrochloric acid solution yields 0.25 M H⁺. In contrast, a 1.2 M strong base like calcium hydroxide produces 2.4 M OH^- due to the generation of two hydroxide ions. The relationship between pH and pOH is defined by the equation pH + pOH = 14, with pH calculated as -log[H⁺] and pOH as -log[OH^-].

Strong Acids and Bases

Recall that strong acids and strong bases completely ionize in water, and because of this fact we can say here that the concentration of H+ ions and the concentration of OH- ions are equal to the concentration of strong acids and bases respectively. So what exactly do I mean by this? Well, here if we take a look, we have 0.25 molar of hydrochloric acid, a strong acid. When it ionizes, it ionizes 100% into these 2 ions. That would mean that the concentration of the acid is the same concentration as my ions, so this would still be 0.25 molar and 0.25 molar. So the concentration of HCl is equivalent to 0.25 molar for H+. For strong bases, they completely ionize as well. They would make 100% of calcium ion and 2 hydroxide ions here. So here we'd have 1.2 molar of calcium ions, but with the hydroxide ion we have to take into account that it's not just 1 hydroxide ion being produced, but 2. What effect does this have on the concentration? Well, we'd say the real concentration of hydroxide ions will be 1.2 molar, the original molarity, times 2, because we make 2 OH- ions. So the complete concentration of hydroxide ion in this case will be 2.4 molarity.

Now, also recall that strong bases may contain the following ions. For a strong base, the 4 basic anions are hydroxide ions, which we saw up above, but also hydride ion, amide ion, and oxide ion. We're going to say when it comes to calculations dealing with pH or pOH, we're going to treat the concentration of OH- ions as being the same as the concentration of hydride ions, amide ions, and oxide ions.

In addition to this, remember the formulas that are connected to pH and pOH. Remember that pH + pOH equals 14. If we know the H+ concentration, that allows us to find pH since it's the negative log of H+. If we know the pH, then we know what H+ concentration is because H+10to the negative pH. pOH equals negative log of OH- concentration, and if you know pOH itself just remember OH-concentration equals10to the negative pOH.

pH of Strong Acids and Bases Example

If the concentration of barium hydride solution is 0.398 molar, calculate its pOH. Barium hydride represents a strong base. Remember, strong bases completely ionize into ions. So, here we form one barium ion plus two hydride ions. The concentration of the strong base is 0.398 molar. Because we produce two hydride ions it'd be 2 times 0.398 molar to get its real concentration. When we do that we get 0.796 molar as the concentration of our hydride ion.

Now, remember that the concentration of hydride ion is equal to the concentration of hydroxide ion. This is important because now that we know the concentration of hydroxide ion, we can find the pOH. pOH itself equals negative log of OH− concentration. So, plug in 0.796, and when we do that, we're gonna get 0.0991 as the pOH of our barium hydride 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

Calculate the pH of a 25 mL of 5.45 × 10⁻² M LiOH solution.

• A. 1.264
• B. 12.736
• C. 0.338
• D. 11.134

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

Here’s what students ask on this topic:

What is the pH of a 0.25 M hydrochloric acid solution?

Hydrochloric acid (HCl) is a strong acid, meaning it completely ionizes in water. For a 0.25 M HCl solution, the concentration of H⁺ ions is also 0.25 M. The pH is calculated using the formula:

$\mathrm{pH}=-\log (H_{+}$

Substituting the concentration:

$\mathrm{pH}=-\log \left(0.25\right)$

Using a calculator, we find:

$\mathrm{pH}=0.60$

Therefore, the pH of a 0.25 M HCl solution is 0.60.

How do you calculate the pOH of a strong base solution?

To calculate the pOH of a strong base solution, you need to know the concentration of OH^- ions. For strong bases, which fully ionize, the concentration of OH^- ions is equal to the base's concentration multiplied by the number of OH^- ions produced per formula unit. The pOH is calculated using the formula:

$\mathrm{pOH}=-\log ({\mathrm{OH}}_{-}$

For example, if you have a 1.2 M solution of calcium hydroxide (Ca(OH)₂), which produces 2 OH^- ions per formula unit, the concentration of OH^- ions is:

${\mathrm{OH}}_{-}$

Then, calculate the pOH:

$\mathrm{pOH}=-\log \left(2.4\right)$

Using a calculator, we find:

$\mathrm{pOH}=0.62$

Therefore, the pOH of the solution is 0.62.

What is the relationship between pH and pOH?

The relationship between pH and pOH is given by the equation:

$\mathrm{pH}+\mathrm{pOH}=14$

This equation holds true for aqueous solutions at 25°C. If you know either the pH or pOH of a solution, you can easily find the other by rearranging the equation:

$\mathrm{pH}=14-\mathrm{pOH}$

or

$\mathrm{pOH}=14-\mathrm{pH}$

For example, if the pH of a solution is 3, the pOH can be calculated as:

$\mathrm{pOH}=14-3=11$

Therefore, the pOH of the solution is 11.

How do you find the concentration of H⁺ ions from pH?

To find the concentration of H⁺ ions from pH, you use the inverse of the pH formula. The pH is defined as:

$\mathrm{pH}=-\log (H_{+}$

To find the concentration of H⁺ ions, rearrange the formula:

$H_{+}$

For example, if the pH of a solution is 4, the concentration of H⁺ ions is:

$H_{+}$

Using a calculator, we find:

$H_{+}$

Therefore, the concentration of H⁺ ions is 1 × 10^-4 M.

What is the pH of a 1.2 M calcium hydroxide solution?

Calcium hydroxide (Ca(OH)₂) is a strong base that fully ionizes in water, producing 2 OH^- ions per formula unit. For a 1.2 M Ca(OH)₂ solution, the concentration of OH^- ions is:

${\mathrm{OH}}_{-}$

The pOH is calculated using the formula:

$\mathrm{pOH}=-\log \left(2.4\right)$

Using a calculator, we find:

$\mathrm{pOH}=0.62$

To find the pH, use the relationship:

$\mathrm{pH}=14-\mathrm{pOH}$

Substituting the pOH value:

$\mathrm{pH}=14-0.62=13.38$

Therefore, the pH of a 1.2 M calcium hydroxide solution is 13.38.