Bronsted-Lowry Definition of acid Base behaviour A Bronsted-Lowry acid is defined as a substance that can donate a proton. A Bronsted-Lowry base is defined as a substance that can accept a proton. HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq) acid base acid base Each acid is linked to a conjugate base on the other side of the equation. Calculating pH pH = – log [H+ ] Where [H+ ] is the concentration of hydrogen ions in the solution. Calculating pH of strong acids Strong acids completely dissociate The concentration of hydrogen ions in a monoprotic strong acid will be the same as the concentration of the acid. For HCl and HNO3 the [H+ (aq)] will be the same as the original concentration of the acid. For 0.1M HCl the pH will be –log[0.1] =1.00 Always give pH values to 2d.p. In the exam Finding [H+ ] from pH [H+ ] = 1 x 10-pH On most calculators this is done by pressing Inv (or 2nd function) log – number(pH) Example 1 What is the concentration of HCl with a pH of 1.35? [H+ ] = 1 x 10-1.35 = 0.045M
22.214.171.124 Brønsted–Lowry acid–base equilibria in aqueous solution (A-level only)
An acid is a proton donor.
A base is a proton acceptor.
Acid–base equilibria involve the transfer of protons.
126.96.36.199 Definition and determination of pH (A-level only)
The concentration of hydrogen ions in aqueous solution covers a very wide range.
Therefore, a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration. pH = –log10[H+]
Students should be able to:
• convert concentration of hydrogen ions into pH and vice versa
• calculate the pH of a solution of a strong acid from its concentration.