Gravimetry

1

• Gravimetry
• Previous Chemistry Courses
• Formulas of Common monatomic and polyatomic ions
  and ionic compounds
• Solubilities of ionic compounds
• Chemical equations for metathesis reactions
  involving precipitate formation
• Metathesis reactions involving gas formation and
  weak electrolyte formation
• Redox equations
• Chemical formulas
• Empirical formulas show the simplest whole number
  ratio of the atoms in a compound
• Molecular formulas give the actual number of
  atoms in a molecule
• CH2O is the empirical formula for formaldehyde
  (CH2O)
• and acetic acid (C2H4O2)
• Structural formulas show the connections between
  atoms and how they are arranged in three space

2

• Gravimetry
• Previous Chemistry Courses
• The mole is the SI unit of amount of matter and
  is defined as the quantity of matter that
  contains the same number of particles as are
  contained in exactly 12 grams of .
• One mole contains Avogadros number of particles,
  6.022 x 1023 particles
• The molar mass is the mass in grams containing
  one mole of substance
• Formula weights are used for ionic substances
• Molecular weights are used for molecular
  substances
• The milli-molar mass is the molar
  mass

3

• Gravimetry
• Previous Chemistry Courses
• Some Stoichiometric relationships
• Blanced chemical equations give combining ratios
  - in moles - of the chemical species involved in
  the reaction
• La(NO3)3(aq ) 3 KF(aq) LaF3(s)
  3KNO3(aq )
• Convert the mass of one substance to the
  equivalent mass of another
• Convert mass of original substance to moles of
  that substance
• Multiply mol of original substance by
  stoichiometric ratio to get moles of desired
  substance
• Multiply mol of desired substance by its molar
  mass to get mass of desired substance

4
Gravimetry Previous courses Example What mass of
KF is required to convert 15.3 g La(NO3)3 to
LaF3? La(NO3)3(aq ) 3 KF(aq)
LaF3(s) 3KNO3(aq )
5

• Gravimetry
• Definition of Gravimetry Gravimetry is a method
  of analysis which is completed by a mass
  determination.
• There are two general kinds of gravimetric
  analyses
• Precipitation method - weigh the analyte or a
  compound of the analyte separated from the
  substances in a mixture as an precipitate
• Volatilization method - decompose a substance
  into a gas
• Capture and determine the mass of the gas
• Weigh the residue and from the mass of the
  original sample and the residue, determine the
  mass of the gas that must have formed
• General gravimetric calculations and the
  Gravimetric Factor
• The percentage of Analyte (A) in a sample with
  known mass
• In some analyses, the numerator is determined
  directly
• Electrolysis of Cu2 to produce Cu(s) on Pt
• Mass of water lost by heating a wet sample

6

• Gravimetry
• General gravimetric calculations and the
  Gravimetric Factor
• The denominator is always known because it is the
  first measurement made in an analysis
• In many cases the numerator is not known.
• A is determined indirectly from the mass of some
  compound of A or a compound formed from A
• The numerator must thus be determined from a
  stoichiometric calculation
• Example What mass of Cl- is stoichiometrically
  equivalent to 1.000 g AgCl?
• Cl- Ag AgCl

Gravimetric Factor