pH scale Logarithmic scale expressing the H + concentration, [H + ]. If the pH changes by a factor of 1, the [H + ] changes by a factor of 10. pH =

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Presentation transcript:

pH scale Logarithmic scale expressing the H + concentration, [H + ]. If the pH changes by a factor of 1, the [H + ] changes by a factor of 10. pH = -log[H + ]pH = -log[H + ]

Water & Self-Ionization H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) –H 3 O + –H 3 O + is called hydronium ion –OH - –OH - is called hydroxide ion interchangeablyIn reality, H + and H 3 O + are used interchangeably. H +H + is called a proton or a hydrogen ion. -

pH of Water Water ionizes to produce both the hydronium ion and the hydroxide ion in equal amounts. Ionization happens to a very small extent [H+] = [OH-] = 1 X M[H+] = [OH-] = 1 X M At this concentration: At this concentration: pH = -log[H + ] = 7

pH to Molarity pH = -log[H + ]. Solve for [H + ]. -pH = log[H + ] Antilog(-pH) = [H + ] Say pH = 5. Then –pH = -5. Antilog(-5) = The –pH becomes the power of 10!The –pH becomes the power of 10!

If the pH = 4If the pH = 4 The [H + ] = ? The pOH = ? The [OH - ] = ? 10 Antilog(-pH) = [H + ] Antilog(-4) = Antilog(-10) = [OH - ] = [OH - ]

Difference in pH = 2, which means the [H + ] differ by 10 2 or 100. The pH of 5 solution has an [H + ] that is 100 X greater than that of the pH 7 solution. If the pH changes from 5 to 7, how do the [H + ] compare?

Significance of pH differences Difference in pH =5 and pH =7 –Difference is 2 units of pH 2 100The [H + ] differ by 10 2 or 100 The pH of 5 solution has an [H + ] that is 100 X greater than that of the pH 7 solution.

pH [H+] 1 M or 0 1X10 0 M 0.1 M or -1 1 X M 0.01 M or -2 1 X M.001 M or -3 1 X M pH0123

Ion Product Constant for Water Value of the equilibrium constant expression for the self-ionization of water. H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) K eq = [H + ] [OH - ] [H 2 O] K w = [H + ] [OH - ] =

If the pH = 2 The [H + ] = ? 1 x M

I f the [H + ] = 1 X M The pH = ? 5 How many time stronger is an acidic solution with A pH of 2 than an acidic solution with a pH of 5? PROBLEM: Relative acid strength 1000x

pOH By analogy, pOH is defined as –log[OH - ] Express [OH - ] in scientific notation If [OH - ] = 1.0 X M Then pOH = -log(10 -3 ) = -(-3) = 3.

If the [OH - ] = 1 X The pOH = ? The pH = ? The [H + ] = ? pOH = -log[OH - ] = -log(10 -3 ) = --3 = 3 14 – 3 = 11 Antilog(-11) = M

I f the [H + ] = 1 X M The pH = ? The pOH = ? The [OH - ] = ?5 14 – 5 = 9 Antilog(-9) = 10 -9

pH + pOH = 14 pH = 3, pOH = pH = 7, pOH = pH = 10, pOH = pH = 9, pOH = pH = 1, pOH =

Acid, Base, or Neutral? All H 2 O contains some H + and some OH - Pure H 2 O – concentrations are very low. [H + ] = [OH - ] = 1 X Molar. When [H + ] = [OH - ], solution is neutral Acidic solution: H + > OH - Basic solution: OH - > H +

[H + ]pH[OH - ]pOHAcidic or Basic M M A B M A M M B

How to safely test pH Instrumental –use a pH meter Indicators –use a series of indicators Reaction –Acid if reacts with metals (other than Cu, Ag, or Au)

Indicator A substance that changes color over a narrow pH range. Use several indicators to narrow down the pH range of the substance.