1) How many moles of oxygen will occupy a volume of 2.5 L at 1.40 atm and 25°C? 2) What volume will 2.0 moles of nitrogen occupy at 720 mm Hg and 25°C?
3) What pressure will be exerted by 25.0 g of CO2 at a temperature of 30°C and a volume of
500 mL?
4) At what temperature will 5.00 g of chlorine gas exert a pressure of 900 mm Hg at a volume
of 650 mL?

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

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.