Calculate the pH for each of the following solutions at 25^∘ C: (a) 0.10 M NH3, (b) 0.050 M C5 H

step 1 So this question asks you what are the pH values for 0 .1m ammonia and 0 .05m purity. So which h

Calculate the pH for each of the following solutions at 25^∘...

Calculate the $\mathrm{pH}$ for each of the following solutions at $25^{\circ} \mathrm{C}:$ (a) $0.10 \mathrm{M} \mathrm{NH}_{3}$, (b) $0.050 \mathrm{M} \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}$
(pyridine). $\left(K_{\mathrm{b}}\right.$ for pyridine $=1.7 \times 10^{-9}$.)

Key Concepts

Acid–Base Equilibrium

Acid–base equilibrium refers to the dynamic balance between acids and bases in solution where proton transfer occurs. Understanding this equilibrium is essential for calculating pH because it governs how substances like weak bases interact with water to form their conjugate acids and hydroxide ions, eventually influencing the solution's overall pH.

Weak Base Dissociation

Weak base dissociation describes the limited reaction of a weak base with water to produce its conjugate acid and hydroxide ion. Unlike strong bases, which dissociate completely, weak bases only partially undergo this reaction, and the extent of dissociation must be determined using the base’s equilibrium constant (Kb).

Equilibrium Constant (Kb)

The equilibrium constant for a weak base, known as Kb, quantifies the extent of its reaction with water. It is the ratio of the concentrations of the products (conjugate acid and hydroxide ion) to the concentration of the undissociated base, and it is pivotal for setting up the equilibrium expression required for pH calculations.

pH and pOH Relationship

The pH and pOH relationship is a fundamental concept in aqueous chemistry, linked by the water dissociation constant at a given temperature. At 25°C, the sum of the pH and pOH is 14, which allows for the conversion between hydroxide ion concentration (from the weak base dissociation) and the hydrogen ion concentration that defines pH.

Approximations in Equilibrium Calculations

For weak bases, the degree of dissociation is typically very small relative to the initial concentration. This allows for common approximations where the change in concentration is considered negligible. Such approximations simplify the mathematical manipulation of the equilibrium expression when calculating the concentration of hydroxide ions and, subsequently, the pH.

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