Calculate Oh and Ph for 0.10 M Nabro

This calculator determines the hydroxide concentration (OH⁻) and pH for a 0.10 M sodium bromide (NaBr) solution. Sodium bromide is a strong electrolyte that dissociates completely in water, making it ideal for demonstrating the relationship between electrolyte concentration and pH.

Introduction

When a strong electrolyte like NaBr dissolves in water, it completely dissociates into its constituent ions. For NaBr, this means:

NaBr (aq) → Na⁺ (aq) + Br⁻ (aq)

The bromide ion (Br⁻) is a weak conjugate base, meaning it doesn't significantly affect the water's autoionization equilibrium. Therefore, the hydroxide concentration (OH⁻) is determined primarily by the water's autoionization constant (Kw).

Calculation Method

The pH of a solution containing a strong electrolyte can be calculated using the following steps:

Determine the initial concentration of the electrolyte (in this case, 0.10 M NaBr).
Since NaBr is a strong electrolyte, it completely dissociates, increasing the concentration of Na⁺ and Br⁻ ions.
The hydroxide concentration (OH⁻) is determined by the water's autoionization equilibrium:

Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C

For dilute solutions (like 0.10 M), the effect of the electrolyte on Kw is negligible, so we can approximate:

[OH⁻] ≈ √Kw = √(1.0 × 10⁻¹⁴) = 1.0 × 10⁻⁷ M

The pH is then calculated as:

pH = -log[H⁺] = -log(√Kw) = -log(1.0 × 10⁻⁷) = 7.0

Note: This calculation assumes the solution is dilute and the electrolyte doesn't significantly affect the autoionization equilibrium.

Example Calculation

Let's calculate the OH⁻ concentration and pH for a 0.10 M NaBr solution:

Given: [NaBr] = 0.10 M
Since NaBr is a strong electrolyte, [Na⁺] = [Br⁻] = 0.10 M
Using Kw = 1.0 × 10⁻¹⁴ at 25°C:

[OH⁻] = √(1.0 × 10⁻¹⁴) = 1.0 × 10⁻⁷ M

Therefore, the hydroxide concentration is 1.0 × 10⁻⁷ M.

Calculating pH:

pH = -log(1.0 × 10⁻⁷) = 7.0

This confirms that a 0.10 M NaBr solution has a neutral pH of 7.0.

Interpretation

The results show that:

The hydroxide concentration is 1.0 × 10⁻⁷ M, which is typical for pure water at 25°C.
The pH is exactly 7.0, indicating a neutral solution.
This demonstrates that the addition of a strong electrolyte like NaBr doesn't significantly affect the pH of dilute solutions.

This calculation is particularly useful for understanding how strong electrolytes behave in aqueous solutions and how their concentration affects solution properties.

FAQ

Why does adding NaBr to water not change the pH?

NaBr is a strong electrolyte that completely dissociates in water. The bromide ion (Br⁻) is a weak conjugate base that doesn't significantly affect the water's autoionization equilibrium, so the pH remains neutral (7.0).

What happens if the NaBr concentration is much higher?

At very high concentrations, the electrolyte could affect the water's structure and the autoionization equilibrium, potentially changing the pH. However, for dilute solutions (like 0.10 M), this effect is negligible.

Is this calculation valid for other strong electrolytes?

Yes, this method applies to any strong electrolyte that doesn't form a significant amount of a weak conjugate base. Examples include NaCl, KCl, and NH₄NO₃.