Calculation of the pH of Strong Acids, Strong Alkalis, Weak Acids and Buffer Solutions

Calculation of the pH of strong acids

Remember that strong acids are completely ionised in water.
The equation pH = -log[H+] can be used to calculate the pH of strong acids. Here, [H+] is the concentration of hydrogen ions.
If the molarity of the acid is known, you can calculate the pH directly using this equation.
Consider as well that for strong monoprotic acids [H+] = the initial concentration of the acid.
For example, if the concentration of a strong monoprotic acid is 0.1M, the pH would be -log(0.1)=1.

Calculation of the pH of strong alkalis

The pH of strong alkalis can be found by first calculating the concentration of hydroxide ions, [OH-].
Remember that strong alkalis dissociate fully into ions in aqueous solutions.
Use the formula [OH-] = (Initial concentration of alkali).
You can then calculate the pOH first using the formula pOH = -log[OH-].
The pH can then be found using the equation 14 - pOH = pH.
For example, if the concentration of a strong monoprotic alkali is 0.1M, the pOH would be -log(0.1) =1, therefore the pH would then be 14 - 1 = 13.

Calculation of the pH of weak acids

Keep in mind that weak acids only partially dissociate in solution.
The dissociation constant of acid (Ka) and the initial concentration are important for these calculations.
A common formula to calculate the hydrogen ion concentration is the quadratic formula, but you can simplify the process by making an assumption that [HA]initial ≈ [HA]equilibrium due to the weak acid not fully dissociating.
You can then solve the hydrogen ion concentration using the formula: [H+] = √(Ka x [HA]initial) and then calculate the pH by using the formula pH= -log[H+].

Calculation of the pH of buffer solutions

Buffer solutions resist changes in pH when small amounts of an acid or an alkali are added.
The pH of buffer solutions can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]).
Remember that [A-] is the concentration of the base or conjugate base (from the salt) and [HA] is the concentration of the weak acid.
You’ll need the pKa value of the weak acid, which can be obtained from the Ka value using pKa=-log(Ka).
For example, if a buffer solution is made up from a weak acid HA with pKa=7 and equal concentrations of HA and A-, the pH of the buffer solution would be pH=7 + log(1) = 7.