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Gravimetric Precipitation and Volatilization Essentials
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1
Question
What are the two major types of gravimetric methods of analysis?
Page 1
Answer
Precipitation methods and volatilization methods
2
Question
In precipitation gravimetry, what is done to the analyte after forming the precipitate?
Page 1
Answer
Filtered, washed free of impurities, converted to a product of known composition by heat treatment, and weighed
3
Question
In volatilization gravimetry, how is the analyte separated?
Page 1
Answer
Converted to a gas of known chemical composition, collected and weighed, or mass determined from sample loss
4
Question
What is the first step in the precipitation method?
Page 1
Answer
Weighing the sample
5
Question
What is the second step in precipitation gravimetry?
Page 1
Answer
Moisture determination
6
Question
What occurs in step 3 of precipitation method?
Page 1
Answer
Conversion of the analyte to a sparingly soluble salt (precipitate)
7
Question
In the precipitation method, what is step 4?
Page 1
Answer
Filtration of the precipitate
8
Question
What is step 5 in the precipitation process?
Page 1
Answer
Washing the precipitate
9
Question
What happens in step 6 of precipitation gravimetry?
Page 1
Answer
Drying or ignition of the precipitate to a form suitable for weighing
10
Question
What is the final step in precipitation method?
Page 1
Answer
Weighing of the converted form of the precipitate
11
Question
In the example for determining Ca content, what reaction occurs in step 3?
Page 1
Answer
Ca²⁺ + C₂O₄²⁻ → CaC₂O₄ (s)
12
Question
In the Ca determination example, what happens during ignition in step 6?
Page 1
Answer
CaC₂O₄ (s) → CaO (s) + CO (g) + CO₂ (g)
13
Question
What is the weighed form in the calcium gravimetric analysis example?
Page 1
Answer
CaO
14
Question
What is a precipitate in gravimetric analysis?
Page 1
Answer
An aggregate of atoms, ions, or molecules greater than 10⁻⁴ cm
15
Question
How do ions in solution form a precipitate?
Page 1
Answer
Ions → colloidal particles (10⁻⁸ cm) → precipitate (>10⁻⁴ cm)
16
Question
Why do colloidal particles resist forming larger precipitates?
Page 1
Answer
They are electrically charged
17
Question
What are the ideal properties of a gravimetric precipitating agent?
Page 1
Answer
Specific/selective reaction, readily filtered/washed, low solubility, unreactive with atmosphere, known composition after drying/ignition
18
Question
Why are precipitates of large particle size desirable in gravimetry?
Page 1
Answer
Easy to filter and wash free of impurities
19
Question
What is a colloidal suspension in precipitates?
Page 1
Answer
Tiny particles (10⁻⁷ to 10⁻⁴ cm) invisible to eye, no settling, hard to filter
20
Question
What is a crystalline suspension?
Page 1
Answer
Temporary dispersion of particles ≥0.1 mm, settles and filters easily
21
Question
What experimental variables influence precipitate particle size?
Page 1
Answer
Precipitate solubility, temperature, reactant concentration, mixing rate
22
Question
What is relative supersaturation or Von Weimarn’s ratio?
Page 1
Answer
(Q - S)/S, where Q is instantaneous concentration, S is equilibrium solubility
23
Question
What happens when relative supersaturation is large?
Page 1
Answer
Precipitate tends to be colloidal
24
Question
What occurs at low relative supersaturation?
Page 1
Answer
Crystalline solid forms (Q low, S high)
25
Question
What are the two pathways of precipitate formation?
Page 1
Answer
Nucleation and particle growth
26
Question
What is nucleation in precipitate formation?
Page 1
Answer
Few ions/atoms/molecules form a stable solid, often on contaminants
27
Question
In particle growth, what competes with nucleation?
Page 1
Answer
Deposition on existing nuclei
28
Question
What results if nucleation predominates?
Page 1
Answer
Large number of small particles
29
Question
What happens if particle growth predominates?
Page 1
Answer
Smaller number of larger particles
30
Question
How does relative supersaturation affect nucleation rate?
Page 1
Answer
Increases enormously with higher supersaturation