Finals

• Theodore W. Richards used it to determine atomic

  weights?

  Gravimetric Analysis
• it is one of the most accurate and precise methods of macro-quantitative analysis?

  Gravimetric Analysis
• The principle involved in gravimetric analysis is based on the law of what?

  law of conservation of mass
• • Quantitative analytical method that involves the determination of the mass of a substance by its chemical conversion to a compound of known composition.
  • The quantitative methods that are based on

  determining the mass of a pure compound to which the analyte is chemically related.

  Gravimeric Analysis
• choose what doesn’t belong in this group?

  1. Analyte is selectively converted to an insoluble form.
  2. The separated precipitate is dried or ignited, possibly to another form, and is accurately weighed.
  3. From the weight of the precipitate and a knowledge of its chemical composition, we can calculate the weight of analyte in the desired form.
  4. None (all of the answers are correct)

  4. None
• what are the fundamental methods of gravimetry?

  Precipitation Gravimetry

  Volatilization Gravimetry

  Electro-gravimetry
• In this method The analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed?

  Precipitation gravimetry
• In this method, The analyte is separated from other

  constituents of a sample by converting it to a

  gas of known chemical composition that can

  be weighed?

  Volatilization gravimetry
• With this method, The analyte is separated by deposition on an electrode by an electrical current?

  Electrogravimetry
• arrange the precipitation method of gravimetry in the right order:

  A.) Analyte is converted to a sparingly soluble

  precipitate.

  B.) Precipitate is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed.

  C.) Analyte must first be converted to a solid (precipitate) by precipitation with an appropriate reagent.

  D.) After cooling, the precipitate is weighed and the

  mass is determined.

  C,A,D,B
• it is the 1st step and this may involve several steps including adjustment of the pH of the solution in order for the precipitate to occur quantitatively and get a precipitate of desired properties, removing

  interferences …etc.

  Preparation of the Solution
• it occurs where every few molecules of precipitate

  aggregate together forming a nucleus?

  nucleation
• This requires addition of a precipitating agent

  solution to the sample solution. Upon addition of the

  first drops of the precipitating agent, what hav yo noticed in the solution?

  supersaturation occurs
• Particle size of precipitates can be predicted by?

  Von Weimarn ratio
• what is the formula of Relative Supersaturation?

  Relative Supersaturation = (Q - S)/S
• What phase is this?

  • The precipitate is left hot (below boiling) for 30 min to 1 hour in order for the particles to be digested.
  • forces the small colloidal particles to agglomerate which decreases their surface area and thus adsorption.

  Digestion of Precipitate
• Washing and Filtering, what do you use to wash the. precipitate?

  dilute volatile electrolyte such as nitric acid,

  ammonium nitrate, or dilute acetic acid.
• (T or F) Filtration should be done in appropriately sized Goosh or ignition ashless filter paper. After the

  solution has been filtered, it should be tested to

  make sure that the analyte has been completely

  precipitated.

  True
• The purpose of this is to remove the

  remaining moisture while the purpose of ignition

  in a muffle furnace is to get a material with

  exactly known chemical structure so that the

  amount of analyte can be accurately

  determined . The precipitate is converted to a

  more chemically stable form?

  Drying and Ignition
• (T or F)The precipitate can be weighed with the

  necessary accuracy in place on the filter paper;

  nor can the precipitate be completely removed

  from the filter paper in order to weigh it?

  False (The precipitate can not)
• • anything unwanted which precipitates with the analyte during precipitation .
  • occurs to some degree in every gravimetric analysis.
  • You cannot avoid it all what you can do is minimize it by careful precipitation and thorough washing

  Coprecipitation
• • Sometimes a precipitate standing in contact with the mother liquor becomes contaminated by the precipitation of an impurity on top of the desired precipitate.
  • To reduce, filter as soon as the precipitation is complete and avoid digestion .

  postprecipitation
• -Ideally a gravimetric precipitating agent should react any with the analyte.

  -Specific reagents which are rare, react only with a double chemical species.

  -Selective reagents which are least common, react with a limited number of species.

  A.) All of the answers are right.

  B.) All of the answers are wrong.

  B.) (check reviewer for right answers)
• S2-, CO32 ,PO43- are usually not selective compared to the organic precipitants but it give precipitates with well known formula?

  Inorganic precipitating agents
• • dimethglyoxime and 8-hydroxyquinoline are more selective than inorganic precipitants.
  • They produce with the analyte less soluble
  • precipitate.
  • They also have high molecular weight so that
  • the weighing error is reduced.
  • The disadvantage of this precipitants is that they usually form with the analyte a precipitate of unknown formula.

  Organic precipitating agents
• represents the weight of analyte per unit weight of

  precipitate. It is obtained from the ratio of the formula weight of the analyte to that of the precipitate, multiplied by the moles of analyte per mole of precipitate obtained from each mole of analyte?

  Gravimetric factor (GF)
• determine whether its an adv or disadv:

  Accurate and precise: Gravimetric analysis is potentially more accurate and more precise than volumetric analysis

  Advantages
• determine whether adv or disadv of gravimetric analysis:

  avoids problems with temperature fluctuations,

  calibration errors, and other problems associated with volumetric analysis?

  Advantage
• determine whether adv or disadv of gravimetric :

  It is an ABSOLUTE method. i.e.

  it involves direct measurements without ant form of calibration.

  Relatively inexpensive:

  Advantages
• determine whether adv or disadv of gravimetric:

  • Proper lab technique is critical
  • Careful and time consuming
  • Very clean glassware

  Disadvantage
• determine whether adv or disadv of gravimetric:

  • Very accurate weighing.
  • Co-precipitation.

  Disadvantages