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General ScienceHardAsked in: CDS · CDS-I 2025, GK

Which one of the following contains all Lewis acids?

  1. A.AlCl3, Co3+, BF3
  2. B.BCl3, NH3, OH-
  3. C.Mg2+, H2O, F-
  4. D.BF3, NH3, H2O

Correct answer

A. AlCl3, Co3+, BF3

Explanation

The correct answer is A, AlCl3, Co3+, BF3. A Lewis acid is a species that accepts a pair of electrons, and all three of these can do so.

G.N. Lewis defined acids and bases in 1923 by electron pairs rather than by protons. Molecules with an incomplete octet are classic Lewis acids: in BF3 and AlCl3 the central boron or aluminium atom has only six electrons in its outer shell and readily accepts a pair, which is why anhydrous AlCl3 is used as a catalyst in Friedel-Crafts reactions. Metal cations such as Co3+, Fe3+ and Mg2+ also accept electron pairs from ligands to form complex ions. Lewis bases, in turn, are electron pair donors with lone pairs, such as NH3, H2O, OH- and F-.

B is wrong because NH3 and OH- are Lewis bases, although BCl3 is an acid. C is wrong because H2O and F- are Lewis bases, although Mg2+ is an acid. D is wrong because NH3 and H2O are Lewis bases.

Exam tip: Lewis acid = electron pair acceptor (BF3, AlCl3, metal ions); Lewis base = electron pair donor (NH3, H2O, OH-).

Practice Questions

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Q2.General ScienceAsked in: CDS · CDS-I 2025, GKEasy

Lactose is made up of :

  1. A.Glucose only
  2. B.Glucose and Fructose
  3. C.Glucose and Sucrose
  4. D.Glucose and Galactose
Show answer
Correct answer: D. Glucose and Galactose

Lactose, the sugar of milk, is a disaccharide formed when one molecule of glucose joins one molecule of galactose. Disaccharides are sugars made of two simple sugar units, or monosaccharides, linked by a glycosidic bond.

Q3.General ScienceAsked in: CDS · CDS-II 2016, GKEasy

In total internal reflection, the light travels from

  1. A.rarer to denser medium and it occurs with no loss of intensity
  2. B.denser to rarer medium and it occurs with no loss of intensity
  3. C.rarer to denser medium and it occurs with loss of intensity
  4. D.denser to rarer medium and it occurs with loss of intensity
Show answer
Correct answer: B. denser to rarer medium and it occurs with no loss of intensity

Total internal reflection happens only when light inside a denser medium meets the boundary with a rarer one at an angle larger than the critical angle, and all of the light is reflected back.

Q4.General ScienceAsked in: CDS · CDS-II 2019, GKEasy

In graphite, each carbon atom is bonded to three other carbon atoms

  1. A.forming a three-dimensional structure
  2. B.in the same plane giving a hexagonal array
  3. C.in the same plane giving a square array
  4. D.in the same plane giving a pentagonal array
Show answer
Correct answer: B. in the same plane giving a hexagonal array

In graphite each carbon atom is joined to three others in a flat layer of hexagonal rings, like a honeycomb.

Q5.General ScienceAsked in: CDS · CDS-I 2022, GKMedium

Consider the following pairs of vaccine and category/type : 1. Covaxin : Inactivated pathogen-based vaccine 2. Covishield : mRNA vaccine 3. Sputnik V : Viral vector-based vaccine Which of the pairs given above is/are correctly matched ?

  1. A.1 only
  2. B.2 and 3 only
  3. C.1 and 3 only
  4. D.1, 2 and 3
Show answer
Correct answer: C. 1 and 3 only

Covaxin is an inactivated virus vaccine and Sputnik V is a viral vector vaccine, but Covishield is a viral vector vaccine, not an mRNA vaccine.