UNIT 6 Solutions

Question 1

Name 2 types of gravimetric analysis

Answer 1

Precipitation gravimetry and volatilization gravimetry

Question 2

What is precipitation gravimetry?

Answer 2

• Analyte is separated from solution of sample as precipitate
• Precipitate is filtered, washed free of impurities
• Converted to compound of known composition that can be weighed

Question 3

What is volatilization gravimetry?

Answer 3

• Analyte is separated from other constituents by converting it to a gas of known chemical composition
• Mass of the gas: measure of analyte concentration

Question 4

Name 5 properties of precipitates.

Answer 4

• Easily filtered out and washed free of contaminants
• Low solubility (don’t want to lose mass during filtration and washing)
• Unreactive with atmosphere
• Known chemical composition after it is dried (rely on molar mass for calculations)

Question 5

What are 2 categories of reagents?

Answer 5

• Specific: react only with a limited # of species, rare

- Selective: react with a limited number of species, more common

Question 6

What is a common selective reagent used? What does it precipitate?

Answer 6

AgNO3

Cl-, Br-, I-, SCN-

Question 7

What type of suspension is easiest to filter?

Answer 7

Crystalline

Question 8

What is the difference between a colloidal suspension and a crystalline suspension?

Answer 8

• Colloidal suspensions: tiny particles, difficult to filter because they don’t settle, 10^-7 to 10^-4 cm in diameter
• Crystalline suspensions: bigger particles, settle spontaneously, easily filtered, > 10^-4 cm in diameter

Question 9

What 4 things is the size of particles influenced by?

Answer 9

• Solubility (low)
• Temp. (solubility increases as temp increases)
• Reactant concentration
• Rate at which reactants are mixed (small batches, slow)

Question 10

What is the size of particles related to?

Answer 10

Relative supersaturation of solution

Question 11

How is relative supersaturaiton calculated?

Answer 11

Relative supersaturation= (Q-S)/S, where S = solubility of precipitate, Q = concentration of solute

Question 12

What kind of relative supersaturation value is preferred? Why?

Answer 12

Small values, it means that particles are bigger - crystalline suspension

Question 13

By what 2 ways are precipitates formed? which is preferable?

Answer 13

nucleation and particle growth

Particle growth is preferable because there are less particles which are larger in size (crystalline)

Question 14

What is digestion?

Answer 14

precipitated heated in solution from which it was formed and allowed to stand in contact with solution

Question 15

What is “peptidization of colloids”?

Answer 15

coagulated colloid reverts to original dispersed state

Question 16

When performing a precipitation, what is a major issue that can arise?

Answer 16

Coprecipitation (other chemical components precipitate in addition to the analyte of interest)

Question 17

What are 3 causes of coprecipitation?

Answer 17

• surface adsorption of the analyte
• mixed crystal formaiton
• occlusion (mechanical entrapment)

Question 18

How can coprecipitation by surface adsorption be avoided?

Answer 18

reprecipitation (filtered solid is redissolved and reprecipitated)

Question 19

How can coprecipitation by mixed crystal formation be avoided?

Answer 19

separating interfering ion before final precipitation step

Question 20

How can coprecipitation by occlusion be avoided?

Answer 20

slow formation rate of precipitation and digestion

Question 21

What substance is often used for homogeneous generation of -OH?

Answer 21

urea

Question 22

What is volatilization gravimetry used for?

Answer 22

Determining water and CO2

Question 23

What is determined with CO2 volatilization gravimetry?

Answer 23

Sodium hydrogen carbonate content of antacid tablet –> treated with H2SO4 to convert sodium hydrogen carbonate to CO2