4. Calculations in Anachem

• Outline


  • SI Units
  • Solutions and their concentrations
  • Chemical stoichiometry
  • Additional problems
• SI


  Acronym for the French "Système International d'Unités"
• Mass


  • An invariant measure of the quantity of matter in an object
  • Weight is the force of attraction between an object and its surroundings, principally the earth
  • A chemical analysis is always based on mass so that the results will not depend on locality
• Mole


  • The SI unit for the amount of a chemical substance
  • Always associated with specific microscopic entities such as atoms, molecules, ions, electrons, other particles, or specified groups of such particles as represented by a chemical formula
  • Avogadro's number NA = 6.022 x 10^23
  • Molar mass M is the mass in grams of 1 mole of that substance
• Millimole


  1/1000 of a mole, and the mass in grams of a millimole, the millimolar mass (mM), is likewise 1/1000 of the molar mass
• Calculating moles and millimoles

  1. Moles: n = m/M

  2. Millimoles: mmol = m/mM
• Sample Problem #1


  • Find the number of moles and millimoles of benzoic acid (M = 122.1 g/mol) that are contained in 2.00 g of the pure acid
• Sample Problem #2


  • What is the mass in grams of Na+ (22.99 g/mol) in 25.0 g of Na2SO4 (142.0 g/mol)?
• Molar concentration


  • The number of moles of a solute species that is contained in 1 liter of the solution (not 1 L of the solvent)
  • Unit is molar, symbolized by M, which has the dimensions of mol/L, or mol-L-1
  • Also the number of millimoles of solute per milliliter of solution
• Sample Problem #3


  • Calculate the molar concentration of ethanol in an aqueous solution that contains 2.30 g of C2H5OH (M=46.07 g/mol) in 3.50 L of solution
• Molar analytical concentration


  • Gives the total number of moles of a solute in 1 liter of the solution
  • Specifies a recipe by which the solution can be prepared regardless of what might happen to the solute during the solution process
• Molar equilibrium concentration


  • The molar concentration of a particular species in a solution at equilibrium
  • Requires knowing how the solute behaves when dissolved in a solvent
• Sample Problem #4


  • Calculate the analytical and equilibrium molar concentrations of the solute species in an aqueous solution that contains 285 mg of trichloroacetic acid, Cl3CCOOH (163.4 g/mol), in 10.0 mL (the acid is 73% ionized in water)
• Preparing solutions


  1. Sample Problem #5: Describe the preparation of 2.50 L of 0.120 M BaCl2 from BaCl2 ⋅ 2H2O (244.3 g/mol)

  2. Sample Problem #6: Describe the preparation of 500 mL of 0.0740 M Cl- solution from solid BaCl2 ⋅ 2H2O (244.3 g/mol)
• Weight percent (w/w)


  Weight solute / weight solution x 100%
• Volume percent (v/v)


  Volume solute / volume solution x 100%
• Weight/volume percent (w/v)


  Weight solute (g) / volume solution (mL) x 100%
• The denominator in each percent concentration expression is the mass or volume of solution rather than mass or volume of solvent
• Weight percent is often used to express the concentration of commercial aqueous reagents
• Weight/volume percent (w/v)


  Weight solute, g / Volume solution, mL x 100%
• The denominator in percent concentration expressions is the mass or volume of solution rather than mass or volume of solvent
• Weight percent


  • 70% (w/w) nitric acid solution
• Volume percent


  • 5% (v/v) aqueous methanol solution
• Weight or volume percent

  • 5% (w/v) aqueous silver nitrate solution
• Parts per million (ppm)

  Concentration expressed as parts per million
• Parts per billion (ppb)

  Concentration expressed as parts per billion
• Parts per thousand (ppt)

  Concentration expressed as parts per thousand
• Calculating molar concentration from ppm

  1. Convert ppm to moles

  2. Divide moles by volume to get molar concentration
• Solution-diluent volume ratio


  Composition specified as volume of concentrated solution : volume of solvent
• Solution-diluent volume ratio


  • 1:4 HCl solution
• 1. value


  Negative logarithm (base 10) of molar concentration
• Calculating p-value


  Take negative logarithm (base 10) of molar concentration
• Calculating molar concentration from p-value


  Take antilog (base 10) of negative p-value
• Density


  Mass per unit volume
• Specific gravity


  Ratio of mass to mass of equal volume of water at 4°C
• Calculating molar concentration from density and weight percent

  1. Convert weight percent to mass

  2. Divide mass by molar mass to get moles

  3. Divide moles by volume to get molar concentration
• Stoichiometry


  Quantitative relationship among amounts of reacting chemical species
• Empirical formula


  Simplest whole number ratio of atoms in a chemical compound
• Molecular formula


  Specifies the number of atoms in a molecule
• Structural formula


  Reveals structural differences between compounds with the same molecular formula