phofsolutions.pdf - pH OF SOLUTIONS OBJECTIVES 1 2 3 4 To...

This preview shows 1 out of 3 pages.

pH OF SOLUTIONS OBJECTIVES 1. To investigate the strengths of acids and bases 2. To examine the effect of concentration on the pH of a solution 3. To examine the effect of salt hydrolysis on pH 4. To determine the properties of buffer solutions INTRODUCTION According to Bronsted/Lowry theory, an acid can be defined as a substance that donates a proton, or hydrogen ion, to another substance. A base is a substance that accepts a proton. In aqueous solution, protons will bond with water molecules to form hydronium ions (H 3 O + ). Thus in an aqueous solution of HCl, HCl is the acid and water is the base. Similarly, in an aqueous solution of acetic acid, CH 3 COOH is the acid and water is the base. But in an ammonia solution, water is the acid and ammonia is the base. Review conjugate acid/base pairs in your book and look carefully at the equations below to be sure you understand this relationship. HCl(aq) + H 2 O(l) 6 H 3 O + (aq) + Cl & (aq) (1) acid 1 base 2 acid 2 base 1 CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO & (aq) (2) acid 1 base 2 acid 2 base 1 NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH & (aq) (3) base 1 acid 2 acid 1 base 2 As you can see, water can act as an acid or a base. It can also undergo a process called auto- ionization , represented by the equation below. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH & (aq) (4) This reaction is an equilibrium, with the equilibrium lying far to the left. Since the concentration of water remains essentially constant, the equilibrium expression can be written as K w = [H 3 O + ][OH & ] (5) where K w equals 1.0 x 10 -14 at 25 o C. In pure water or in any neutral aqueous solution at 25 o C, the concentration of H 3 O + ions must be equal to the concentration of OH & ions. Both are 1.0 x 10 -7 M. In an acidic solution , [H 3 O + ] > [OH & ]. In a basic or alkaline solution , [OH & ] > [H 3 O + ]. However, the product must always be 1.0 x 10 -14 at a temperature of 25 o C. Scientists often report the acidity of a solution in terms of pH , where pH is defined as the negative logarithm of the hydronium ion concentration, or pH = & log 10 [H 3 O + ] (6)
Image of page 1

Subscribe to view the full document.