What is the first step in calculating the pH of a weak acid solution?

The first step in calculating the pH of a weak acid solution involves establishing the equilibrium expression. This is essential because weak acids do not completely dissociate in solution; instead, they reach an equilibrium between the undissociated acid and the ions produced. To understand how much of the weak acid dissociates, we need to set up the equilibrium expression based on the dissociation equation of the weak acid.

The equilibrium expression for a weak acid ((HA)) dissociating into (H^+) and (A^-) can be represented as:

[

K_a = \frac{[H^+][A^-]}{[HA]}

]

where (K_a) is the acid dissociation constant. To calculate the pH accurately, the concentrations of all species at equilibrium must be known, requiring the establishment of this expression.

While identifying the solute present is important, it comes before establishing the equilibrium expression. Measuring the temperature of the solution is less relevant for the initial step in calculating pH, though it can influence the dissociation constant. Creating a dilution table is not necessary as a first step if the equilibrium is understood; instead, it may become useful when determining the concentrations used in the equilibrium expression