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Self-ionization of Water and pH Objectives: 1. Identify the ions present in pure water and give their concentrations. 2. Define pH and explain how the.

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Presentation on theme: "Self-ionization of Water and pH Objectives: 1. Identify the ions present in pure water and give their concentrations. 2. Define pH and explain how the."— Presentation transcript:

1 Self-ionization of Water and pH Objectives: 1. Identify the ions present in pure water and give their concentrations. 2. Define pH and explain how the pH scale is used. 3. Identify what a buffer is and how the strength of it is related to the buffer capacity.

2 Water has many unusual properties. One of these properties is water's ability to act as either an acid or a base. (H 2 O  H 3 O + + OH - ) Amphoteric is the name given to a substance that can act as either an acid or a base. Water becomes hydronium and hydroxide ions through self ionization – The concentrations of each for pure water are equal at a pH of 7

3 Ion-Product The ion-product (K w ) can be used to find the concentration of acid and base in any solution based on the following assumptions: – The formula for K w for ALL solutions at 25 o C is K w = [H 3 O + ][OH - ] = 1x pH of pure water is 7, so the concentration of H 3 O + is 1 x10 -7 The concentration of base therefore is [OH - ] = 1x /1x = 1 x10 -7 – EX: If a solution has 1 x10 -2 H 3 O + ions, what is the concentration of the base ions? [OH - ] = 1x /1x = 1 x10 -12

4 pH Scale The pH scale was developed to help people interpret the amount of hydronium ions compared to the amount of base in a solution. The scale was developed by Soren Sorenson, a Danish biochemist, and is based on the -log of the hydronium ion (H 3 O + ) concentration in solution.{-log[H 3 O + ]}. – The pH scale contains values between 0 and14. – Each whole number change in the pH of a substance represents a 10 fold increase in the number of hydronium ions. – Scale is from acidic (solutions with a higher hydronium ion (H 3 O + )concentration than 1 x10 -7 are acidic ; 0 most acidic) 7 neutral 7-14 basic (solutions with a higher hydroxide ion (OH - )concentration than 1 x10 -7 are basic ; 14 most basic) – can be measured using a chemical indicator a weak acid or base that changes color as the pH changes (gain or lose H + ions)

5 pH Scale pH scale can be used to determine the amount of hydronium and hydroxide concentrations in solution with the equation pH + pOH = 14 and the ion product (K w ). Consider the following examples. example: A solution with 1 x H 3 O + ions, what is the pH and pOH of the solution? – to get the pH simply change the negative exponent to a positive pH pH = 2 pOH = =12 A solution with 1 x OH - ions Use the ion product constant 1st to find the concentration of hydronium – K w =1 x /1 x OH - = 1 x H 3 O + ions pH = 12 pOH = 2


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