# Hydrogen Ions and Acidity The Ionization of Water and pH.

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Hydrogen Ions and Acidity The Ionization of Water and pH

Objectives When you complete this presentation, you will be able to: describe the autoionization of water determine the hydrogen ion concentration in water when given the hydroxide ion concentration in water determine the hydroxide ion concentration in water when given the hydrogen ion concentration in water determine pH or pOH of a solution when given the hydrogen ion or hydroxide ion concentration determine the hydrogen ion or hydroxide ion concentration when given the pH or pOH

Hydrogen Ions in Water We are used to thinking of water as a pure liquid that contains only H 2 O molecules. However, as a pure liquid, water ionizes into hydrogen ions and hydroxide ions: H 2 O(l) → H + (aq) + OH - (aq) H 2 O(l) + H 2 O(l) → H 3 O + (aq) + OH - (aq)

Hydrogen Ions in Water The symbol for the concentration of hydrogen ions is [H + ] and for hydroxide ions is [OH - ]. We can measure the concentration of the ions in pure water. [H + ] = 1.0×10 -7 M [OH - ] = 1.0×10 -7 M By stoichiometry, H 2 O(l) → H + (aq) + OH - (aq), we see that [H + ] = [OH - ] in pure water.

Hydrogen Ions in Water We also know that the product of the concentrations is a constant, K w. K w = [H + ][OH - ] = 1.0×10 -14 This means that as we change [H + ], we change [OH - ]. If [H + ] goes up, [OH - ] goes down. If [H + ] goes down, [OH - ] goes up.

Hydrogen Ions in Water The higher the value of [H + ] and lower the value of [OH - ], the more acidic the solution. The lower the value of [H + ] and higher the value of [OH - ], the more basic the solution. For example: [H + ] = 1.0×10 -6 is acidic. For example: [H + ] = 1.0×10 -3 is more acidic. For example: [H + ] = 1.0×10 -8 is basic. For example: [H + ] = 1.0×10 -11 is more basic.

Hydrogen Ions in Water Example 1: Colas are slightly acidic. If a cola solution has [H + ] = 1.0×10 -5 M, what is the [OH - ]? [H + ] = 1.0×10 -5 M K w = [H + ][OH - ] = 1.0×10 -14 K w = [H + ][OH - ] [OH - ] = KwKw [H + ] = 1.0×10 -14 1.0×10 -5 [OH - ] = 1.0×10 -9 M

The pH Concept pH is another way to measure [H + ] and/or [OH - ]. pH = -log[H + ] pOH = -log[OH - ] If [H + ] = 1.0×10 -7 then pH = -log(1.0×10 -7 ) = 7.00 If [OH - ] = 1.0×10 -7 then pOH = -log(1.0×10 -7 ) = 7.00

The pH Concept To find pH on the TI-83 or TI-84 Plus … press the “(-)” button press the “LOG” button enter the value for [H + ] press the “)” button press “ENTER”

The pH Concept To find pH on the TI-Nspire … press the “(-)” button press the “ctrl” button press the “10 x ” button enter the value for [H + ] press the “)” button press “ENTER” NOTE: The position of these buttons on your calculator may change with faceplate.

The pH Concept If [H + ] = 1.0×10 -5 then pH = -log(1.0×10 -5 ) = 5.00 If [OH - ] = 1.0×10 -8 then pOH = -log(1.0×10 -8 ) = 8.00 If [H + ] = 3.1×10 -2 then pH = -log(3.1×10 -2 ) = 1.51 If [OH - ] = 4.5×10 -11 then pOH = -log(4.5×10 -11 ) = 10.35

The pH Concept Find the pH of a solution that has 1.[H + ] = 1.0×10 -4 2.[H + ] = 1.0×10 -9 3.[H + ] = 2.4×10 -5 4.[H + ] = 6.6×10 -8 5.[H + ] = 4.8×10 -3 pH = 4.00 pH = 9.00 pH = 4.62 pH = 7.18 pH = 2.32

The pH Concept Find the pOH of a solution that has 1.[OH - ] = 1.0×10 -10 2.[OH - ] = 1.0×10 -3 3.[OH - ] = 3.8×10 -5 4.[OH - ] = 8.1×10 -8 5.[OH - ] = 2.8×10 -3 pOH = 10.00 pOH = 3.00 pOH = 4.42 pOH = 7.09 pOH = 2.55

The pH Concept We can also convert from pH and pOH to [H + ] and [OH - ] respectively. [H + ] = 10 -pH [OH - ] = 10 -pOH If pH = 7.00 then [H + ] = 10 -7.00 = 1.0×10 -7 If pOH = 3.25 then [OH - ] = 10 -3.25 = 5.6×10 -4

The pH Concept To find [H + ] on the TI-83 or TI-84 Plus … press the “2ND” button press the “LOG” button press the “(-)” button enter the value for pH press the “)” button press “ENTER”

The pH Concept To find pH on the TI-Nspire … press the “10 x ” button press the “(-)” button enter the value for pH press the “)” button press “ENTER” NOTE: The position of these buttons on your calculator may change with faceplate.

The pH Concept If pH = 2.556 then [H + ] = 10 -2.556 = 2.78×10 -3 If pOH = 9.27 then [OH - ] = 10 -9.27 = 5.4×10 -10 If pH = 12.12 then [H + ] = 10 -12.12 = 7.6×10 -13 If pOH = 6.678 then [OH - ] = 10 -6.678 = 2.10×10 -7

The pH Concept Find the [H + ] of a solution with a 1.pH = 6.00 2.pH = 13.00 3.pH = 5.25 4.pH = 9.42 5.pH = 2.32 [H + ] = 1.0×10 -6 [H + ] = 1.0×10 -13 [H + ] = 5.6×10 -6 [H + ] = 3.8×10 -10 [H + ] = 4.8×10 -3

The pH Concept Find the [OH - ] of a solution with a 1.pOH = 4.00 2.pOH = 11.00 3.pH = 3.79 4.pH = 10.46 5.pH = 3.77 [OH - ] = 1.0×10 -4 [OH - ] = 1.0×10 -11 [OH - ] = 1.6×10 -4 [OH - ] = 3.5×10 -11 [OH - ] = 1.7×10 -4

The pH Concept pH + pOH = 14 if pH = 2.0, then pOH = 14.0 - 2.0 = 12.0 if pH = 3.250, then pOH = 14.000 - 3.250 = 10.750 if pOH = 3.0, then pH = 14.0 - 3.0 = 11.0 if pOH = 7.451, then pH = 14.000 - 7.451 = 6.549

The pH Concept Solutions with pH = 7 (pOH = 7) are neutral. Litmus paper remains colorless. Solutions with pH 7) are acidic. Litmus paper turns red. Solutions with pH > 7 (pOH < 7) are basic. Litmus paper turns blue.

Measuring pH Indicators may be added to a solution to tell us the range of pH of the solution.

Measuring pH Indicators are most often used where pH may change, such as in a titration. But they are also used to tell us where we are in a particular range of pH. For example, if we wanted to tell if a solution has a pH greater than or less than 7... we might use Bromthymol Blue (which changes color at about pH = 7).

Measuring pH Indicators are most often used where pH may change, such as in a titration. But they are also used to tell us where we are in a particular range of pH. For example, if we wanted to tell if a solution has a pH greater than or less than 7... we would not want to use Methyl Red (which changes color near pH = 5).

Measuring pH Some meters provide quick and accurate readings of pH. They vary from the simple to the very complex.

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