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

## 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.