How many atoms of iron are in 0.750 moles of the compound
iron(iii) oxide?

2.26 × 10²⁴ atoms

Explanation:

In 1 mole of Fe₂O₃ (Iron(III) oxide), there are 6.02 × 10²³ molecules of Fe₂O₃ (Avogadro's number). The molecules of Fe₂O₃ in 0.750 moles are:

0.750 mol × (6.02 × 10²³ molecules/ 1 mol) = 4.52 × 10²³ molecules

In 1 molecule of Fe₂O₃, there are 5 atoms (2 atoms of Fe and 3 atoms of O). The atoms in 4.52 × 10²³ molecules of Fe₂O₃ are:

4.52 × 10²³ molecule × (5 atom/1 molecule) = 2.26 × 10²⁴ atom

There are 2.26 × 10²⁴ atoms in 0.750 moles of Fe₂O₃.

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hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.