Which one of the following is a covalent crystal?

• Rock salt

• Ice

• Quartz

• Dry ice

Which one of the nitrogen atoms in $\underset{\mathrm{I}}{{\mathrm{H}}_2\mathrm{N}} - \underset{\mathrm{II}}{\mathrm{NH}} - \mathrm{CO} - \underset{\mathrm{III}}{{\mathrm{NH}}_2}$ is the most nucleophilic?

• III

• I

• II

• All three nitrogen atoms are equally strong nucleophilic centres

The maximum number of possible optical isomers in 1-bromo-2-methyl cyclobutane is

• 4

• 2

• 8

• 16

Which one of the following is the most energetic conformation of cyclohexane?

• Boat

• Twisted boat

• Chair

• Half-chair

Which one of the following is an intermediate in the reaction of benzene with CH₃Cl in the presence of anhydrous AlCl₃?

• Cl^-

• CH${}_3^{-}$

• CH${}_3^{+}$

• 

Formic acud is a stronger acid than acetic acid. This can be explained using

• +M effect

• -I effect

• +I effect

• -M effect

The IUPAC name of 

• but-3-enoic acid

• but-1-enoic-acid

• pent-4-enoic acid

• prop-2-enoic acid

38.

Which one of the following statements is false?

• During roasting, moisture is removed from the ore

• The ore is freed from almost all non-metallic impurities

• Calcination of ore is carried out in the absence of any blast of air

• The concentrated zinc blende is subjected to calcination during its extraction by pyrometallurgy

In chromite ore, the oxidation number of iron and chromium are respectively

• +3, +2

• +3, +6

• +2, +6

• +2, +3

The number of naturally occurring p block elements that are diamagnetic is

• 18

• 6

• 5

• 7