2nd Exam definitions (3)

Question 1

Major types of gravimetric methods of analysis (2)

Answer 1

Precipitation methods - analyte converted to a soluble precipitate then converted to a product of known composition and weighed

Volatilization Methods - Analyte is separated into a gas and product is determined from loss of mass in sample

Question 2

Precipitation methods

Answer 2

1. Weighing the sample
2. Moisture determination
3. Conversion of the analyte to
   a sparingly soluble salt (precipitate)
4. Filtration of the precipitate
5. Washing the precipitate
6. Drying or ignition of the precipitate (conversion to a
   form suitable for weighing)
7. Weighing of the converted form of the precipitate.

Question 3

Ideal gravimetric precipitating agent and analyte product

Answer 3

1. readily filtered and washed free with contaminants.
2. of sufficiently low solubility so that no significant loss of the analyte occurs during filtration and washing.
3. unreactive with the constituents of the atmosphere.
4. of known composition after it is dried, or if necessary
   ignited.

Question 4

particle size of a precipitate is influenced by

Answer 4

precipitate solubility,

temperature,

reactant concentration,

rate at which reactants are mixed.

Question 5

Supersaturated solution effects (2)

Answer 5

when relative supersaturation is large, precipitate tends to be colloidal.

when relative supersaturation is small (Q is low and S is
high), a crystalline solid is more likely to form.

Question 6

Precipitates form by two different pathways (2)

Answer 6

Nucleation - molecules come together to form a stable solid. Often on contaminants. Particle growth can be hampered through competition.

Question 7

Colloidal suspension is joined together to a filterable solid

Coagulated colloid reverts back to dispersed state

Answer 7

coagulation

Peptization

Question 8

Crystalline precipitates

Answer 8

Yields a purer and more filterable product

Question 9

Minimizing Q in crystalline solids

Maximizing S in crystalline solids

Answer 9

Using dilute sol’ns and adding precipitating reagent w good mixing

Precipitating from hot solution or adjusting pH

Question 10

Coprecipitation occurs when soluble compounds get removed during precipitate formation

4 types

Answer 10

Equilibrium processes
Surface adsorption - contamination of precipitates with large surface areas.

Mixed-Crystal formation - ions of a solid is replaced by ion of another element

Kinetics of crystal growth
Occlusion - crystal grows rapidly leading to trappin gof ions

Mechanical entrapment - crystals lie close together during growth. Crystals grow together and trap a portion of solution in a tiny pocket

Question 11

Factors

Answer 11

Colloidal - High supersaturation - Nucleation (contaminants) - High Q Low S

Crystalline - Low supersaturation - Particle Growth (Existing nuclei) - Low Q High S