• The acidity of a solution varies on the concentration of H+ (aq) and is measured along the pH scale

pH = -log10[H+(aq)]

  • On a pH scale:
    • The smaller the pH, the greater the concentration of H+(aq)
    • A difference of one pH increments means there is a tenfold difference in H+

i.e. pH 3 has 10x the amount of Hcompared to pH 4

Using Kw to determine pH of a base

  • At a known temperature,
  • At 298K, the Kw =[H+(aq)] [OH(aq)] = 1.0 x 10-14 mol2 dm-6

[H+(aq)] = [OH(aq)] = 1.0 x 10-7 mol dm-3 

pH = -log10[H+(aq)] = -log10 [1.0 x 10-7] = 7.00

pH = 7.00

Calculating pH from Hydrogen ion concentration

  • If the hydrogen ion concertation is given the pH can be calculated by using the formula: pH = -log10[H+(aq)]

e.g. A solution of Nitric acid has a hydrogen ion concentration of 0.01 mol dm-3

pH = -log10[H+(aq)] = -log10[0.01] = 1

pH= 1

Calculating Hydrogen Ion Concentration from pH

  • To find the hydrogen ion concentration from a given pH then the inverse of the pH formula is used: [H+] 10-pH

e.g. A solution of hydrochloric acid has a pH of 1.52

[H+] = 10-pH = 10-1.52 = 0.03 mol dm-3

Calculating Hydroxide Ion Concentration from pH

  • The Concentration of OH ions is proportional to the concentration of the base, for example if there is a 0.04 mol dm-3 sodium hydroxide solution there will be 0.04 mol dm-3of [OH]
  • However, to find the pH the [H+] must be known, therefore the ionic product of water(Kw) is used

Kw = [H+][OH]

  • If the [OH] for a strong aqueous base and Kw at a certain temperature are both known then the [H+] can be determined and therefore the pH

e.g. The value of Kw at 298K is 1.0 x 10-14 mol2 dm-6. Determine the pH of 0.1 mol dm-3 NaOH at 298K

[OH] = 0.1 mol dm-3 à [H+] = Kw

[OH]

= 1.0 x 10-14 = 1.0 x 10-13 mol dm-3

0.1

pH = -log10 = 1.0 x 10-13 = 13.0

pH and Temperature

  • The pH of water can vary depending on its temperature

H2O(l) ⇋ H+(aq) + OH(aq)  ΔH = + 57.3kJ mol-1

  • As the reaction is endothermic, by increasing the temperature the reaction will favour the right side, thus producing more H+ and OH
  • As there are more H+ ions, the pH will increase to 6
  • However, this does not mean the solution is more acidic, as although there are more H+ ions, there is in equal proportions OH ions which cancel the H+ ions out

Temperature (K)

Kw (mol2 dm‑6)

pH

273

0.114 x 10-14

7.47

283

0.293 x 10-14

7.27

293

0.681 x 10-14

7.08

298

1.008 x 10-14

7.00

303

1.471 x 10-14

6.92

313

2.916 x 10-14

6.77

323

5.476 x 10-14

6.63

373

51.30 x 10-14

6.14

Strong Monoprotic Acids

  • Monoprotic refers to each molecule of an acid which will release 1 proton when it is dissociated
  • Examples of monoprotic acids are Hydrochloric and Nitric acid
  • Both HCl and HNO3 will produce 1 mole of hydrogen ions, therefore the concentration of H+ is equal to the acids concentration
  • g. 0.1 mol dm-3 of HCl is [H+] = 0.1 moldm-3

pH = log10 [0.1] = 1.0

Strong Diprotic Acids

  • Diprotic acids are those which release 2 protons when they dissociate
  • Examples of diprotic acids are Sulfuric acid and carbonic acid
  • Diprotic acids produce two moles of hydrogen ions for each mole of acid, therefore there is twice the concentration of Hthan the acid itself
  • g. 0.1 mol dm-3 of H2SO4 is [H+] = 0.2 mol dm-3

pH = log10 [0.2] = 0.70