AP Chemistry Experimental Procedures Quick Guide

Gravimetric Analysis

• Weigh sample, dissolve in water.
• Add chemical to form precipitate.
• Filter to collect precipitate.
• Rinse precipitate to remove soluble ions.
• Repeated heating and weighing until constant mass is obtained.
• Common mistakes:
  • Precipitate not dry: mass is water, percent yield too high.

Making a Solution

• Weigh solute on analytical balance.
• Add solvent to volumetric flask.
• Swirl until solute dissolves.
• Add solvent to the mark on the volumetric flask.
• Molarity $=$ moles solute/L of solution
• Common mistakes:
  • Solid stuck in flask neck: Dissolve in beaker first.
  • Overfilling flask: dilute solution
  • Not using distilled water
  • Not using a volumetric flask: Loss of precision in concentration of prepared solution

Titration

• Fill buret with titrant (solution of known concentration).
• Use pipette to transfer analyte in flask.
• Add indicator to analyte.
• Titrate until indicator changes color (endpoint).
• Molarity $=$ moles solute/L of solution
• $M<em>1V</em>1 = M<em>2V</em>2$ for monoprotic acids at equivalence point.
• Equivalence point: moles of acid $=$ moles of base.
• Endpoint: indicator changes color.
• Common mistakes:
  • Overshooting titration: analyte concentration appears higher.
  • Not using indicator: No perceivable endpoint.
  • Incorrect indicator: pH at equivalence point should be approximately equal to the $pK_a$ of the indicator.
  • Improper buret cleaning: titrant is dilute, analyte appears more concentrated

Beer's Law

• Use spectrophotometer to analyze concentration of solutions.

• Pick wavelength where absorbance is highest.

• Measure absorbance for different concentrations and graph.

• $A = \epsilon b c$

  • A = absorbance
  • \epsilon = molar absorptivity (constant)
  • b = path length of cuvette (cm)
  • c = concentration (molarity)

• Common mistakes:

  • Low absorbance: cuvette was wet, too little solute.
  • High absorbance: dirty cuvette, too much solute, wrong wavelength.

Chromatography

• Separate components of a mixture based on polarity.
• $R_f = \frac{distance \, travelled \, by \, component}{distance \, travelled \, by \, solvent}$
• Paper is relatively nonpolar compared to solvent.
• Substance that travels further is more attracted to solvent.
• Substance that travels least is most attracted to paper.
• Compare $R_f$ values between trials.
• Common mistakes:
  • Solvent reaches top of paper: cannot calculate $R_f$
  • No polarity difference: inadequate separation

Fractional Distillation

• Separate components based on boiling point differences.
• Distillate: substance with lower boiling point.

Coffee Cup Calorimetry

• Used to solve for specific heat/heat of reaction.
• $q = mC\Delta T$
  • q = heat (J or calories)
  • m = mass of solution (reactants + water) or object (g or kg)
  • C = specific heat capacity
  • $\Delta T = T<em>{final} - T</em>{initial}$
  • $\Delta H_{rxn} = \frac{q}{moles \, reactant \, used}$
• Endothermic: decrease in temperature.
• Exothermic: increase in temperature.
• Water part of surrounding and not the system.
• Common mistakes:
  • Not stirring enough.
  • Lid not sealed tightly on calorimeter
  • Endothermic reaction temperature doesn't change enough

Solution Preparation by Dilution

• $M<em>1V</em>1 = M<em>2V</em>2$
• Add acid to water.
• Common mistakes:
  • Overfilling volumetric flask: dilute solution.
  • Not using distilled water.
  • Not using a volumetric flask: Loss of precision in concentration of prepared solution

Gas Collection Over Water

• Collect gases formed in reactions.
• Use room temperature water.
• Read gas volume where water level is equal.
• $P<em>{atmosphere} = P</em>{gas} + P_{water \, vapor}$

Percent Composition/Formula of a Hydrate

• Heat hydrate to drive off water.
• Moles of anhydrous salt : moles of water = ratio for hydrate.
  • $CuSO<em>4 \cdot 5H</em>2O(hydrated) \rightleftharpoons CuSO<em>4(anhydrous) + 5H</em>2O$
• Common mistakes:
  • Not heating enough: fewer moles of water.
  • Overheating: salt decomposes, appears more water.
  • Salt spills: appears more water.
  • Crucible weighed warm: inaccurate mass.