chemistry : class12 Chemical Concentration : Concentration of ores : General Principles and Processes of Isolation of Elements MCQ Questions and Answers
Practice important
Chemical Concentration
MCQ questions and answers.
Page 1
of
1
|
10
MCQ Questions
1. In the leaching of bauxite by the Bayer process, which condition most effectively dissolves alumina while leaving ferric oxide as an insoluble residue?
A.
Aqueous NaOH under pressure at elevated temperature
B.
Dilute HCl at room temperature
C.
Aqueous NaCl under pressure at elevated temperature
D.
Concentrated H2SO4 at room temperature
Correct Answer:
Aqueous NaOH under pressure at elevated temperature
Explanation:
Bauxite is treated with concentrated NaOH solution under pressure and elevated temperature. Amphoteric Al2O3 dissolves as soluble sodium aluminate, whereas Fe2O3 remains largely undissolved and can be filtered off.
2. During chemical concentration of bauxite, the species formed when alumina dissolves in excess aqueous sodium hydroxide is best represented as:
A.
AlCl3
B.
NaAlO2 or [Al(OH)4]− in solution
C.
Al2(SO4)3
D.
Al(OH)3 precipitate only
Correct Answer:
NaAlO2 or [Al(OH)4]− in solution
Explanation:
Alumina is amphoteric and reacts with aqueous NaOH to form soluble aluminate species. Depending on the representation used, the dissolved product is written as NaAlO2 or [Al(OH)4]−.
3. In the cyanide leaching of a gold-containing ore, why is gold selectively dissolved while many gangue materials remain undissolved?
A.
Gold forms a stable soluble dicyanoaurate complex in the presence of oxygen
B.
Gold forms an insoluble gold oxide that settles separately
C.
Gangue minerals are converted into volatile cyanides
D.
Gold is physically attracted to cyanide ions and floats
Correct Answer:
Gold forms a stable soluble dicyanoaurate complex in the presence of oxygen
Explanation:
In cyanide leaching, oxygen assists oxidation of gold, and cyanide stabilizes it as the soluble complex [Au(CN)2]−. Most gangue constituents do not form comparable soluble complexes under these conditions.
4. Which net reaction correctly represents the dissolution of gold in an aerated aqueous cyanide solution during chemical concentration?
Explanation:
Gold is oxidized and stabilized by cyanide as [Au(CN)2]−, while oxygen is reduced in alkaline solution. The balanced overall reaction is 4Au + 8CN− + O2 + 2H2O → 4[Au(CN)2]− + 4OH−.
5. A zinc ore containing soluble zinc compounds and substantial iron(III) oxide gangue is treated with aqueous sodium hydroxide. Which property enables selective chemical concentration of zinc?
A.
Zinc forms soluble zincate, whereas iron(III) oxide remains insoluble
B.
Zinc forms an insoluble hydroxide, whereas iron(III) oxide forms soluble ferrate
C.
Both zinc and iron form volatile hydroxides
D.
Zinc is reduced to metallic zinc by sodium hydroxide
Explanation:
Zinc hydroxide and related zinc compounds are amphoteric and dissolve in excess NaOH to form soluble zincate species, such as [Zn(OH)4]2−. Iron(III) oxide does not dissolve appreciably under these leaching conditions.
6. In the purification stage of the Bayer process, why is aluminium hydroxide precipitated from sodium aluminate liquor by seeding and cooling?
A.
Aluminium hydroxide becomes less soluble under these conditions, while sodium hydroxide remains in solution
B.
Sodium hydroxide decomposes into aluminium hydroxide and sodium metal
C.
Ferric oxide reacts with water to form aluminium hydroxide
D.
Aluminium hydroxide is oxidized to metallic aluminium in the liquor
Correct Answer:
Aluminium hydroxide becomes less soluble under these conditions, while sodium hydroxide remains in solution
Explanation:
After insoluble impurities are removed, dilution, cooling, and seeding promote hydrolysis and precipitation of Al(OH)3 from sodium aluminate solution. The caustic soda remains largely in solution and can be recycled.
7. Which sequence correctly describes the chemical concentration of gold ore by cyanide leaching followed by recovery of gold from the leach liquor?
A.
Formation of [Au(CN)2]−, adsorption or precipitation of gold from the solution, and regeneration of metallic gold
B.
Formation of AuCl4−, filtration of gold chloride, and direct sublimation
C.
Formation of Au(OH)3, flotation of the hydroxide, and dehydration to gold
D.
Formation of Au2S, magnetic separation, and roasting to gold
Correct Answer:
Formation of [Au(CN)2]−, adsorption or precipitation of gold from the solution, and regeneration of metallic gold
Explanation:
Cyanide leaching first converts gold into soluble [Au(CN)2]−. Gold is then recovered from the pregnant solution, commonly by zinc cementation or adsorption followed by elution and reduction.
8. Why is the cyanide concentration and alkalinity carefully controlled during chemical concentration of gold ores?
A.
To maintain formation of the soluble gold cyanide complex while reducing cyanide loss and formation of hazardous HCN
B.
To convert all gangue minerals into soluble chlorides
C.
To ensure that gold remains as an insoluble oxide
D.
To prevent oxygen from participating in gold dissolution
Correct Answer:
To maintain formation of the soluble gold cyanide complex while reducing cyanide loss and formation of hazardous HCN
Explanation:
Cyanide must be available to stabilize dissolved gold as [Au(CN)2]−. Alkaline conditions suppress conversion of CN− into volatile and highly toxic HCN, while appropriate cyanide levels limit reagent wastage.
9. A mixed ore contains an amphoteric oxide of the desired metal and a basic oxide impurity. Which reagent is most suitable for chemical concentration when the desired oxide dissolves in alkali but the impurity does not?
A.
Excess aqueous NaOH followed by filtration
B.
Dilute aqueous NaCl followed by filtration
C.
Liquid mercury followed by distillation
D.
Aqueous KNO3 followed by crystallization
Correct Answer:
Excess aqueous NaOH followed by filtration
Explanation:
Selective leaching relies on different chemical behavior toward the reagent. An amphoteric desired oxide dissolves in excess NaOH as a soluble hydroxo or aluminate-type complex, while a basic oxide impurity generally remains undissolved and is removed by filtration.
10. Which feature distinguishes chemical concentration by leaching from ordinary physical concentration methods?
A.
The desired mineral is selectively converted into a soluble chemical species using a suitable reagent
B.
The ore is separated solely according to differences in particle density
C.
The ore is separated solely according to magnetic susceptibility
D.
The valuable mineral is separated solely by differences in surface wettability without reaction
Correct Answer:
The desired mineral is selectively converted into a soluble chemical species using a suitable reagent
Explanation:
Chemical concentration by leaching depends on selective chemical reaction and dissolution of the valuable component or its impurity. The dissolved species is then separated from insoluble gangue by filtration or a subsequent precipitation step.
Short Questions and Answers
Practice short questions and answers related to
Chemical Concentration
.
Short questions are currently unavailable for this page.