INTRODUCTION: Cell Composition & Buffers and Titration

• What is the pH of a solution consisting of 0.15M NH4Cl and 1.5M NH3? (The Kb of NH3 is 1.8x10^-5)

  Answers (round off to 2nd decimal place):

  pKa = 9.26

  pH = 10.26

  Condition:

  HA < A- thus, pH > pKa

  10.26 > 9.26
• What is the pH of a solution consisting of 0.75M HC2H3O2 and 0.50M NaC2H3O2? (The Ka of HC2H3O2 is 1.8x10^-5)

  Answers (round off to 2nd decimal place):

  pKa = 4.74

  pH = 4.57

  Condition:

  HA > A- thus, pH < pKa

  4.57 < 4.74
• Calculate the pH of a solution containing 15g of HF and 21g of NaF in 750mL of solution. (The Ka of HF is 7.2x10^-4)

  Answers (round off to 2nd decimal place):

  pKa = 3.14

  pH = 2.97

  Condition:

  HA > A- thus, pH < pKa

  2.97 < 3.14
• What is the pKa of an unknown weak acid if the pH of the solution was measured to be 5.62 when [HA] = 0.450M and [A-] = 0.850M?

  Answers (round off to 2nd decimal place):

  pKa = 5.34

  Condition:

  HA < A- thus, pH > pKa

  5.62 > 5.34
• If the initial volume of 0.5M NaOH is 50ml and the final volume is 30ml used to neutralize 30ml HCl completely. What is the concentration of HCl?

  0.33M
• The components of the cell can be classified in different ways

  • Polar vs Non-polar
  • Organic vs Inorganic
• polar - able to dissolve completely in water due to dipole-dipole interactions, ion-dipole, or H-bonding
• London dispersion forces - occur between adjacent nonpolar molecules forming temporary dipole interactions and are extremely weak
• Polarizability ↑ = larger electron cloud = larger surface area
• Dipole-dipole forces - occur between polar molecules forming permanent dipole interactions
• Hydrogen bonds - strongest form of dipole-dipole force wherein hydrogen bond with a more electronegative atom
• Ion-dipole forces - occur between ions and polar molecules. (i.e. table salt in water)
• Coulomb force - the force of attraction or repulsion between two like and unlike charges
• Coulomb's law tells us that the electrostatic attraction between ion and dipole is directly related to the magnitudes of the ion charge and the dipole, and inversely related to the distance between them
• Arrange the intermolecular forces from strongest to weakest:

  ion-dipole > hydrogen bonding > dipole-dipole > dipole-induced dipole > London dispersion forces
• non-polar - unable to dissolve in water (i.e. lipids and waxes)
• organic - compounds with carbon and hydrogen. i.e. four major macromolecules
• inorganic - ionic compounds or electrolytes usually devoid of carbon and hydrogen
• inorganic - usually involved in cell signalling, enzyme activity, and cell regulation
• buffers - aqueous systems that resist changes in pH when small amounts of acid (H+) or base (OH-) are added
• Different forms of buffers:

  • Weak acid with conjugate base
  • Proteins or amino acids
• weak acids - gives off protons (H+), producing H3O+
• • Acid - gives H+ → oxidized
  • Base - receives H+ → reduced
• • Monoprotic: gives off 1 H+
  • Polyprotic: gives off >1 H+
  • Diprotic: gives off 2H+ (i.e. H2CO3, alanine)
  • Triprotic: gives off 3H+ (i.e. H3PO4)
• • HF/NaF
  • When + Acid (adding acid to buffer sol’n):
  • ACID: H3O+
  • CONJUGATE BASE reacts & accepts H+ from added acid:

  H3O+ + F → HF + H2O

  • w/o weak base, added acid increases poH = decrease pH, making the sol’n more acidic
• • HF/NaF
  • When + Base:
  • BASE: OH-
  • WEAK ACID reacts & donates H+ to added base
  • OH- + HF → F + H2O
  • w/o weak acid, added base decreases pOH = increase pH, making the sol’n more basic
• Henderson-Hasselbalch Equation:

  ph=pka+log([A-]/[HA])
• At isoelectric point, the concentration of the weak acid is equal that of the conjugate base in the solution

  • [A-]/[HA]=1
  • pH = pKa + log[1]
  • log[1]=0

  →  pH = pKa 
• • To CREATE A BUFFER SOL’N:
  • pH = pKa
• If the ratio of base & acid is 10:1

  log(10) = 1 (then pH will be 1 unit away from the pKa)

  ADD: base > acid

  SUBTRACT: acid > base
• Determine the pH of the buffer solution conceptually:

  Given:

  • [A-] = 1M
  • [HA] = 0.1
  • pKa = 4

  pH = 5
• • [HA] > [A-] ; pH < pKa
  • [A-] > [HA] ; pH > pKa
  • [A-] = [HA] ; pH = pKa
• pH - measure of hydrogen ions
• pH formula:

  • -log[H3O+] or -log[H+]
• pOH - measure of hydroxide ions
• pOH formula:

  • -log[OH-]
• • FORMULAS TO REMEMBER IN CALCULATING PH:
  • Henderson-Hasselbalch Equation:

  pH = pKa + log([A-]/[HA])

  • pKa = -log[Ka]
  • pKb = -log[Kb]
  • pKa+pKb = 14

  Moles to mass: grams/1 x 1mole/grams = mole
• amino acids - buffer system in the blood
• amino acids has both carboxyl group (ACID) & amino group (BASIC)
• Concentration of Solution - the number of particles of the solute dissolved in a given solvent