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Acids and bases, pH and buffers

Acids and bases, pH and buffers. Dr. Mamoun Ahram Lecture 2. Acids and bases. Acids versus bases. Acid: a substance that produces H+ when dissolved in water (e.g., HCl, H 2 SO 4 ) Base: a substance that produces OH - when dissolved in water (NaOH, KOH) What about ammonia (NH 3 )?.

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Acids and bases, pH and buffers

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  1. Acids and bases, pH and buffers Dr. Mamoun Ahram Lecture 2

  2. Acids and bases

  3. Acids versus bases • Acid: a substance that produces H+ when dissolved in water (e.g., HCl, H2SO4) • Base: a substance that produces OH- when dissolved in water (NaOH, KOH) • What about ammonia (NH3)?

  4. Brønsted-Lowry acids and bases • The Brønsted-Lowry acid: any substance able to give a hydrogen ion (H+-a proton) to another molecule • Monoprotic acid: HCl, HNO3, CH3COOH • Diprotic acid: H2SO4 • Triprotic acid: H3PO3 • Brønsted-Lowry base: any substance that accepts a proton (H+) from an acid • NaOH, NH3, KOH

  5. Acid-base reactions • A proton is transferred from one substance (acid) to another molecule Ammonia (NH3) + acid (HA)  ammonium ion (NH4+) + A- • Ammonia is base • HA is acid • Ammonium ion (NH4+) is conjuagte acid • A- is conjugate base

  6. Water: acid or base? • Both • Products: hydronium ion (H3O+) and hydroxide

  7. Amphoteric substances • Example: water NH3 (g) + H2O(l) ↔ NH4+(aq) + OH–(aq) HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)

  8. Acid/base strength

  9. Rule • The stronger the acid, the weaker the conjugate base HCl(aq)→ H+(aq) + Cl-(aq) NaOH(aq)→ Na+(aq) + OH-(aq) HC2H3O2 (aq) ↔ H+(aq) + C2H3O2-(aq) NH3 (aq) + H2O(l)↔ NH4+(aq) + OH-(aq)

  10. Equilibrium constant    HA  <-->   H+ + A- Ka: >1 vs. <1

  11. Expression • Molarity (M) • Normality (N) • Equivalence (N)

  12. Molarity of solutions moles = grams / MW M = moles / volume (L) grams = M x vol (L) x MW

  13. Exercise • How many grams do you need to make 5M NaCl solution in 100 ml (MW 58.4)? grams = 58.4 x 5 moles x 0.1 liter = 29.29 g

  14. Normal solutions N= n x M (where n is an integer) • n =the number of donated H+ Remember! The normality of a solution is NEVER less than the molarity

  15. Exercise • What is the normality of H2SO3 solution made by dissolving 6.5 g into 200 mL? (MW = 98)?

  16. But… • Molarity (and normality) is not useful for understanding neutralization reactions. 1M HCL neutralizes 1M NaOH But… 1M HCl does not neutralize 1M H2SO3 • Why? 

  17. Equivalents • The amount of molar mass (g) of hydrogen ions that an acid will donate • or a base will accept • 1 mole HCl = 1 mole [H+] = 1 equivalent • 1 mole H2SO4 = 2 mole [H+] = 2 equivalents

  18. Examples • One equivalent of Na+ = 23.1 g • One equivalent of Cl- - 35.5 g • One equivalent of Mg+2 = (24.3)/2 = 12.15 g

  19. Exercise • calculate the number of equivalents of:40g of Mg+216g of Al+3 Mg++ : 40g x (1mol/24g) x (2eq/1mol) = 3.3 eq Al3+: 16g x (27g/1mol) x (3eq/1mol) = 1.8 eq

  20. Exercises • Calculate milligrams of Ca+2 in blood if total concentration of Ca+2 is 5 mEq/L. (MW = 40.1) (20.1 g/1000 mEq) x (1000 mg/g) x (5 mEq /L) = 100 mg/L • What is the normality of H2SO3 made by dissolving 6.5g in 200 ml? equivalents? (MW = 98)

  21. Examples (calculate grams)

  22. Titration and equivalence point • The concentration of acids and bases can be determined by titration

  23. Excercise • A 25 ml solution of 0.5 M NaOH is titrated until neutralized into a 50 ml sample of HCl. What is the concentration of the HCl? Step 1 - Determine [OH-] Step 2 - Determine the number of moles of OH- Step 3 - Determine the number of moles of H+ Step 4 - Determine concentration of HCl

  24. A 25 ml solution of 0.5 M NaOH is titrated until neutralized into a 50 ml sample of HCl • Moles of base = Molarity x Volume • Moles base = moles of acid • Molarity of acid= moles/volume

  25. Another method MacidVacid = MbaseVbase

  26. Note • What if one mole of acid produces two moles of H+ • Consider the charges (or normality)

  27. Modified equation Na x Va= Nb x Vb Na = normality of acid Va = volume of acid Nb = normality of base Vb = volume of base NaOH=1 H2SO3=2 H3PO4=3

  28. Exercises • If 19.1 mL of 0.118 M HCl is required to neutralize 25.00 mL of a sodium hydroxide solution, what is the molarity of the sodium hydroxide? • If 12.0 mL of 1.34 M NaOH is required to neutralize 25.00 mL of a sulfuric acid, H2SO4, solution, what is the molarity of the sulfuric acid?

  29. Ionization of water H3O+ = H+

  30. Equilibrium constant Keq = 1.8 x 10-16 M

  31. Kw • Kw is called the ion product for water

  32. pH

  33. What is pH?

  34. Exercise • What is the pH of 0.01 M HCl? • What is the pH of 0.01 N H2SO3? • What is the pH of a solution of 1 x 10-11 HCl?

  35. Acid dissociation constant • Strong acid • Strong bases • Weak acid • Weak bases

  36. pKa

  37. What is pKa?

  38. Henderson-Hasselbalch equation

  39. The equation pKa is the pH where 50% of acid is dissociated into conjugate base

  40. Buffers

  41. Maintenance of equilibriumLe Châtelier’s principle

  42. What is buffer?

  43. Titration

  44. Midpoint

  45. Buffering capacity

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