Aqueous Solutions and pH Hydronium Ions and Hydroxide Ions Acids produce the hydronium (H3O+) ion in water: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) Bases produce the hydroxide ion (OH-) in water: NaOH(aq) Na+(aq) + OH-(aq) Self-Ionization of Water Water produces both hydronium and hydroxide ions through self- ionization: H2O(l) + H2O(l) H3O+(aq) + OH-(aq) (Brackets [ … ] are used to symbolize concentration in molarity) Kw = ionization constant of water Kw = [H3O+] [OH-] (Kw = 1 x 10-14 M2) [H3O+] = 1 x 10-7M [OH-] = 1 x 10-7M pH Scale - A means of expressing acidity or basicity of a solution. http://www.mpcfaculty.net/mark_bishop/pH_range.jpg pH Expressing acidity or basicity in terms of H3O+ or OH- concentration can be difficult due to extremely small values. pH is a more convenient means of quantitatively describing acidity of a solution. pH stands for “pouvoir hydrogene” – power of hydrogen. pH Calculations pH = -log[H3O+] The logarithm (log) of a number is equal to the power that 10 must be raised to equal the number. Example: log (1 x 10-7) = -7 pH Practice 1) What is the pH of a solution with a [H3O+] = 1.0 x 10-7? pH = -log(1 x 10-7) pH = -(-7.0) = 7.0 2) What is the pH of a solution with a [H3O+] = 3.21 x 10-4? pH = -log (3.21 x 10-4) pH = - (-3.49) = 3.49 pOH Calculations & Practice pOH = -log[OH-] 1) What is the pOH of a solution with a [OH-] = 1.0 x 10-9? pOH = -log(1 x 10-9) pOH = -(-9.0) = 9.0 2) What is the pOH of a solution with a [OH-] = 6.74 x 10-8? pOH = -log (6.74 x 10-8) pOH = - (-7.17) = 7.17 Calculating Concentrations from pH & pOH [H3O+] = 10-pH [OH-] = 10-pOH 1) What is the [H3O+] of a solution with a pH = 12.3? [H3O+] = 10-12.3 [H3O+] = 5.01x10-13M 2) What is the [OH-] of a solution with a pOH = 4.25? [OH-] = 10-4.25 [OH-] = 5.62x10-5M Connection between hydronium & hydroxide pH + pOH = 14 [H3O+] [OH-] = 1 x 10-14 Neutral solutions: [H3O+] = [OH-] & pH = 7 Acidic solutions: [H3O+] > [OH-] & pH < 7 Basic solutions: [H3O+] < [OH-] & pH > 7 Summary of Equations pH = -log[H3O+] pOH = -log[OH-] [H3O+] = 10-pH [OH-] = 10-pOH pH + pOH = 14 [H3O+] [OH-] = 1 x 10-14 Practice Chart [H3O+] [OH-] pH pOH 6.7 x 10-4 3.17 12.2 2.5 x 10-8 Calculator Help If you have a TI-30Xa such as the one shown, you must enter the information in the formula backwards. For example: What is the pH of a solution with a [H3O+] = 3.21 x 10-4? 3 2 1 pH = - log (3.21 x 10-4) 1) Enter 3.21EE-4 2) Press “log” 3) Press +/- key Calculator Help cont. What is the [H3O+] of a solution with a pH = 12.3? 3 2 1 [H3O+] = 10 -12.3 1) Enter 12.3 2) Press +/- key 3) Press 10x key Graphing and 2-line calculators Enter exactly as formula reads!
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