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Ionization of Water

pH. Ionization of Water. When compounds dissociate/ionize in and aqueous solution, they produce ions - hydronium ( H 3 O + ) and hydroxide ( OH - ) T hese ion concentrations can be expressed more easily as [H 3 O + ] = 1.0 x 10 - 7 M and [OH - ] = 1.0 x 10 - 7 M.

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Ionization of Water

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  1. pH

  2. Ionization of Water • When compounds dissociate/ionize in and aqueous solution, they produce ions - hydronium (H3O+) and hydroxide (OH-) • These ion concentrations can be expressed more easily as [H3O+] = 1.0 x 10-7 M and [OH-] = 1.0 x 10-7 M. • The brackets indicate “concentration of” in Molarity (M).

  3. Kw • The product of [H3O+] and [OH-] is equal to 1.0 x 10-14 M2, which is a constant known as the ionization constant of water, KW. • Because the [H3O+] and [OH-] in pure water are equal, pure water is neutral. • Any substance whose [H3O+] does not equal its [OH-] is going to be acidic or basic.

  4. Calculating Kw • To calculate either the acid of base concentration, you must know how to rearrange the KW equation.

  5. Example • What is the [H3O+] if [OH-] = 4.56 x 10-8 M? • [H3O+] = ? • [OH-] = 4.56 x 10-8 M

  6. pH Scale pH of Common Substances

  7. pH Equations • A more convenient way of expressing the acidity (or basicness) of a solution is pH and pOH. pH = -log[H3O+] pOH = -log[OH-] pH + pOH = 14 pH values from 0 - 6.9 are acidic 7.1 - 14 are basic 7.0 is neutral pOH values from 0 - 6.9 are basic 7.1 - 14 are acidic 7.0 is neutral.

  8. pH Example • What is the pH of 0.050 M HNO3? pH = -log[H3O+] pH = -log[0.050] pH = 1.3 Acidic or basic? Acidic

  9. pOH Example • What is the pOH of 0.070 M NH3? Is it acidic or basic? pH = -log[OH-] pOH = -log[0.070] = 1.15 It is acidic because 1.15  7.

  10. Example pH (pOH) calculations • If you had a solution with the concentration of 0.025 M [H3O+], what is the solutions pH? What is the solutions pOH? Given: [H3O+] = 0.025 M Unknown: pH = ? pH = -log [H3O+] = -log[0.025M] = 1.6 pOH = 14 – pH = 14 - 1.6 = 12.4

  11. Example pH (pOH) calculations • What is the pH of a 1.5 M solutions of KOH? (this is a base) Given: [1.5 M] Unknown: pH = ? pOH = - log [OH-] pOH = - log [1.5] pOH = -0.18 pH + pOH = 14 14 – pOH = pH 14- (-0.18) = 14.18 pH = 14.18 Check : Answer does indicate that KOH forms a sol’n pH  7, which is basic.

  12. Converting pH & pOH to Concentrations • If you want to know the [H3O+] or [OH-] from a given pH or pOH value, use the following equations: • Example: What is the [OH-] if the pH = 12.27 ? • pH = 12.27 • pOH = ? • [OH-] = ? [H3O+] = 10-pH

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