pH Calculation after mixing acid and base

  • Calculate the pH of the resultant solution when solutions of acids and/or bases are diluted or mixed

Mixing acids and Bases

Example 1

Calculate the pH after 30 mL of 0.10 mol L–1 sodium hydroxide is added to 50 mL of 0.30mol L­–1 hydrochloric acid solution. Include a balanced equation in your answer.

$$HCl (aq) + NaOH(aq) \rightarrow NaCl(aq)+ H_2O(l)$$
Moles of NaOH added =
$$n = cV = 0.10 \times 0.030 = 0.0030 mol$$
Moles of HCl present =
$$n = cV = 0.30 \times 0.050 = 0.015 mol$$
Since 1 mol HCl reacts with 1 mol NaOH, then all the NaOH will be neutralised by the HCl
Mol NaOH remaining = 
$$0.015 - 0.0030 = 0.012mol$$
Volume solution = 
$$30 + 50 = 80mL$$
Remaining [HCl] =
$$\frac{0.012}{0.08} = 0.15M$$

Since HCl is a strong acid, [H+] = [HCl] = 0.15 M


pH = -log(0.15) = 0.82 (2 s. f.)