pH

The Ion product constant for water H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH − (aq) Dissociation of water is an equilibrium, and since liquid water has no effect, we can calculate the ion product constant for water: Kw = [H 3 O + ][OH − ]

The Ion product constant for water Very few (2/billion) water molecules dissociate. The [H3O + ] and [OH − ] were determined to be 1.0 x mol/L each. Kw =1.0×10 −7 mol/L × 1.0×10 −7 mol/L = 1.0 ×10 −14

The Ion product constant for water Since we know that at equilibrium the K for water (Kw) is 1.0 x we can determine the [H3O + ] and [OH − ] if one is known.

Problem Find [H3O + ] and [OH − ] in each solution. (a) 2.5 mol/L nitric acid (b) 0.16 mol/L barium hydroxide You know that nitric acid is a strong acid and barium hydroxide is a strong base. Since both dissociate completely in aqueous solutions, you can use their molar concentrations to determine [H3O+] or [OH−]. You can find the concentration of the other ion using Kw.

Practice Problems P 566 #12-15

The pH Scale Strong acids contain 10 mol/L of H 3 O + Strong bases contain mol/L of H 3 O + Since these numbers are inconvenient we use the pH scale to simplify. Logarithms are used to calculate pH

Logarithms The log of any number is the power to which you must raise 10 to equal that number: log 10 10=1 because 10 1 =10 log =2 because 10 2 =100 Log =3 because 10 3 =1000

Calculating pH pH= - log[H 3 O + ] Sometimes [H + ] is used as an abbreviation. p stands for exponential “power” of 10 [H 3 O + ] or [H + ] is the concentration of acid in mol/L

Calculating pH

Problem Calculate the pH of a solution with [H 3 O + ] = 3.8 × 10 −3 mol/L.

Practice P

Calculating pOH pOH refers to the ion concentration of hydroxide. Calculate pOH using the [OH - ] pOH= - log[OH - ] Since Kw= [H3O + ][OH − ]= 1.0x Then, pH + pOH= 14

Problem A liquid shampoo has a hydroxide ion concentration of 6.8 ×10 −5 mol/L at 25°C. (a) Is the shampoo acidic, basic, or neutral? (b) Calculate the hydronium ion concentration. (c) What is the pH and the pOH of the shampoo?

Using antilogs You can calculate [H 3 O + ] or [OH − ] by finding the antilog of the pH or pOH. [H 3 O + ] = 10 −pH [OH − ] = 10 −pOH To use the antilog, push 2 nd and then log on your calculator. It should give you 10 ^( then insert –pH or –pOH

Problem If the pH of urine is outside the normal range of values, this can indicate medical problems. Suppose that the pH of a urine sample was measured to be 5.53 at 25°C. Calculate pOH, [H3O+], and [OH−] for the sample.

Practice P 572 # Section Review: P 578 # 1-5, and 8, especially 8 Quiz Friday May 2 nd Acid/Base Properties pH/pOH [H 3 O + ] / [OH - ]