# 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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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.

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

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 - ] = 1x10 -14 pH of pure water is 7, so the concentration of H 3 O + is 1 x10 -7 The concentration of base therefore is [OH - ] = 1x10 -14 /1x 10 -7 = 1 x10 -7 – EX: If a solution has 1 x10 -2 H 3 O + ions, what is the concentration of the base ions? [OH - ] = 1x10 -14 /1x 10 -2 = 1 x10 -12

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 0 - 14 0-7 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)

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 10 -2 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 = 14 -2 =12 A solution with 1 x 10 -2 OH - ions Use the ion product constant 1st to find the concentration of hydronium – K w =1 x 10 -14 /1 x 10 -2 OH - = 1 x 10 -12 H 3 O + ions pH = 12 pOH = 2

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