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8.2: pH of a Solution

8.2: pH of a Solution. The pH is a way of indicating the hydrogen ions (H + (aq) ) present in sol n. What does water look like? H 2 O (l) → H + (aq) + OH - (aq) Water is neutral, which means that that: [H + (aq) ] = 1 x 10 -7 mol/L. pH of a Solution.

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8.2: pH of a Solution

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  1. 8.2: pH of a Solution The pH is a way of indicating the hydrogen ions (H+ (aq)) present in soln What does water look like? H2O (l)→ H+(aq) + OH- (aq) Water is neutral, which means that that: [H+ (aq)] = 1 x 10-7 mol/L

  2. pH of a Solution Since water is neutral and its [H+(aq)] = 1 x 10-7 mol/L Then what does an acid and base look like? Acidic Soln: [H+(aq)] > 1 x 10-7 mol/L Basic Soln: [H+(aq)] < 1 x 10-7 mol/L

  3. pH of a Solution So… what does [H+(aq)] = 1 x 10-7 mol/L mean? And how does that help me find the pH? Well… [H+(aq)] = 10-pH How does this help me find anything? Well… that is quite simple by using the following equation: pH = -log [H+(aq)]

  4. Some problems 1. If [H+] = 0.053 mol/L, calculate the pH pH = -log [H+(aq)] pH = -log [0.053] pH is expressed as 2 decimal places pH = 1.28 2. If [H+] = 2.45 x 10-8 mol/L, calculate the pH pH = -log [H+(aq)] pH = -log [2.45 x 10-8] pH = 7.61

  5. Some problems 3. If pH = 1.45, calculate the [H+] [H+] = 10-pH = antilog (-pH) [H+] = 10-1.45 = antilog (-1.45) [H+] = 0.035 mol/L 4. If pH = 9.15, calculate the [H+] in mol/L [H+] = 10-pH = antilog (-pH) [H+] = 10-9.15 = antilog (-9.15) [H+] = 7.1 x 10-10 mol/L

  6. Logarithm? What is that? A Logarithm is an exponent which uses the base 10 the log of 100 = 2 (because 102 = 100) the log of 1000 = (103) = 3 the log of 158 = (102.2) = 2.2 the log of 1 = 0 (because 1 = 100) the log of 0.01 = -2 (because of 10-2 = 0.01) the log of 0.0001 (10-4) = -4 the log of 0.05012 (10-1.3) = -1.3

  7. pH Examples

  8. Changing the pH? When a solution is diluted the pH will change. However, pH is on factor of ten scale. If a 1 mL soln at pH = 1 is diluted with 9 mL of water the pH will change to ? 2 Once again, you may be asking yourself… “Self, how else can we change the pH?”

  9. Neutralization Neutralization is a rxn between an acid and a base that results in a pH closer to 7 H2SO4 (aq) + 2 KOH (aq) → 2 H2O (l) + K2SO4(aq) Let’s do some more examples on the board…

  10. How to Measure pH? • Different types of pH paper • pH meter, the one shown here is what is used in undergraduate labs

  11. Practice Questions: Page 374 # 8, 9 Page 375 # 1, 2, 5

  12. Acid Safety A A A Always Add Acid

  13. Chemical Indicators for pH Phenophthalein: Colourless from pH 0 to pH 8 Pink between pH 8 and pH 10 Bromothymol blue Yellow from pH 6 to pH 7.6 Blue from pH 7.6 and up

  14. Bronsted-Lowry Theroy An acid is a proton (H+ ion) donator A base is a proton (H+ ion) acceptor In the equation you will have: HBr(aq) + H2O(l) → H3O+(aq) + Br-(aq)

  15. Bronsted-Lowry Theroy What is H3O+(aq) ???? This is called a Hydronium ion. They are formed when acids are in soln. Amphiprotic: A substance that acts as an acid or a base (gain or lose a proton), HCO3-(aq), H2O(l), HSO3-(aq), H2PO4-(aq), HPO42-(aq) HCO3-(aq) + H3O+(aq) → H2CO3(aq) + H2O(l) HCO3-(aq) + HO-(aq) → CO32-(aq) + H2O(l)

  16. Conjugate acid-base pair Conjugate acid-base pair Bronsted-Lowry Theroy What does this look like in soln? Acid Base Conjugate acid Conjugate Base HBr(aq) + H2O(l) → H3O+(aq) + Br-(aq)

  17. Comparisons: Turn to page 388, look at summary Arrhenius HA(aq) → H+(aq) + Cl-(aq) Bronsted-Lowry HA(aq) + H2O(l) → H3O+(aq) + Cl-(aq)

  18. Titration: A lab procedure involving the addition of solution from a buret into a sample Example: Titrating a base into an acid End Point: The point in a titration at which a sharp change in a property occurs Example: neutralization in an acid-base reaction

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