CHAPTER 3

• Ways to measure


  • Mass
  • Volume
  • Counting pieces
• Measure mass


  In grams
• Measure volume


  In liters
• Count pieces


  In moles
• Atomic, Molecular and Formula Weight

  • Gram-atomic weight is the weight of a specified number of atoms of that element
  • Gram-molecular weight is the sum of the atomic weights of the atoms that make up a compound
  • Gram-formula weight or molar mass is a more accurate description for substances that exist as ionic compounds
• Moles


  • Moles (g/mol) = grams / formula weight
  • ONE mol of substances = atomic/molecule/formula weight of the substances
• Molarity


  Molarity (M) is the concentration of a solution expressed as the number of moles of solute per liter of solution
• Normality


  • Normality (N or eq/L) = mass (g) / (eq.wt x V (L))
  • Equivalent wt (g/eq) = molecular wt / no.reacting unit
• Reaction types and their equivalent units

  • Precipitation: Charge of cation or anion
  • Acid-base: Number of H+ donated by acid or accepted by base
  • Complexation: Number of electron accepted by metal or donated by ligand
  • Oxidation-reduction: Number of electron released by reducing agent or accepted by oxidizing agent
• Molality


  • A one-molal solution contains one mole per 1000 g of solvent
  • Molality is commonly used in physicochemical measurements because it is not temperature dependent as molar and normal
• Common Units for Reporting Concentration

  • Wt/wt, Wt/vol, Vol/vol, ppt, mg/g, g/kg, mg/mL, g/L, ppm, mg/kg, µg/g, mg/L, µg/mL, µL/L, nL/mL, ppb, µg/kg, ng/g, µg/L, ng/mL, nL/L, pL/mL, %
• Density


  Amount of mass present in a given volume
• Specific Gravity


  Ratio of the density of a substance to the density of a reference substance
• Dilutions


  • The desired molarity solutions are often prepared from concentrated stock solutions by adding water.
  • Moles of solute before dilution = Moles of solute after dilution
  • M1 x V1 = M2 x V2
• The ice floats on top of the water
• Salad oil is less dense than vinegar
• Density


  • Gas = low density
  • Liquids: close to 1 g/cm3, 1 g/mL
  • Metals: various heavy densities
• Density Example 1


  Given: Gas fills a volume of 1200 mL and has a mass of 1.60 g

  Calculate: Density of the gas

  Density = mass / volume = 1.60 g / 1200 mL = 0.00133 g/mL
• Density Example 2


  Given: A cube of pure silver measures 2.0 cm on each side, density of silver is 10.5 g/cm3

  Calculate: Mass of the cube
• Density Example 3


  Given: Density of air is 1.25 x 10-3 g/cm3, room dimensions are 5.00 m x 4.00 m x 2.2 m

  Calculate: Mass of air in the room
• Dilution


  A process where the concentration of a solution is lowered by adding solvent to the solution without adding more solute
• Making a Dilution


  Remove sample from concentrated solution

  Add solvent to make dilute solution

  Shake to mix
• Dilution Example


  Given: 94.0% (g/100 g) H2SO4 with density 1.831 g/mL, need to prepare 1 L of 0.100 M solution

  Calculate: Volume (mL) of 94.0% H2SO4 required

  Molarity of 94.0% H2SO4 solution = 17.5 M

  Use formula: M1V1 = M2V2 to calculate V1 = 5.7 mL
• Dilution Example 2


  Given: 98.0% (wt/wt) H2SO4 with concentration 18.0 M, need to dilute to 0.1 M H2SO4 in 1.00 L

  Calculate: Volume (mL) of 98.0% H2SO4 required

  Density of 98.0% H2SO4 needs to be calculated first
• Titration


  A technique where the analyte reacts with a known concentration titrant (standard solution) from a burette. The volume of titrant required to just completely react with the analyte is measured.
• Titration Principles


  Write reaction equation and find ratio of reactants

  Calculate moles of titrant from volume and concentration

  Use conversion factor to find moles of analyte: molA = molB * a/b
• Titration Example


  Given: 0.4671 g sample containing sodium bicarbonate, titrated with 0.1067 M HCl, requiring 40.72 mL

  Reaction: NaHCO3 + HCl → NaCl + H2CO3

  Calculate: Percent sodium bicarbonate in the sample
• Primary Standard


  A highly pure material used to prepare a standard solution by dissolving an accurately weighed quantity and diluting to an accurately known volume
• Secondary Standard


  A solution standardized by titrating a primary standard, less accurate than a primary standard due to titration errors
• Requirements of a Primary Standard

  • 100.00% pure

  Stable to drying and at room temperature, always dried before weighing

  Readily available

  High formula weight to reduce weighing error
• Types of Volumetric Methods

  • Acid-Base (neutralization reaction)

  Precipitation (titrant forms insoluble product with analyte)

  Complexometric (titrant is a complexing agent forming water-soluble complex with analyte)

  Oxidation-Reduction (titration of an oxidizing agent with a reducing agent)
• Yield of Chemical Reactions

  Theoretical yield (calculated from equation) is the calculated quantity of product

  Actual yield (from experiment) is the amount actually obtained

  Percent yield = (actual yield / theoretical yield) * 100
• Limiting Reactant


  The reactant which runs out first, determines the amount of product

  Excess Reactant is added to ensure the limiting reactant is completely used up
• Finding Limiting Reactant


  Calculate the amount of product produced by each reactant

  The reactant that produces the lesser amount of product is the limiting reactant
• Calculating Excess Reactant


  Calculate moles of limiting reactant

  Calculate moles of excess reactant that reacts