Question

The isomers butane and methylpropane have(1) the same molecular formula and the same properties
(2) the same molecular formula and different properties
(3) a different molecular formula and the same properties
(4) a different molecular formula and different properties

Answer 1

Your answer is (2) the same molecular formula and different properties, because structural isomers usually have different properties to their parent. 

Answer 2

Answer: (2) the same molecular formula and different properties

Explanation: Butane ($C_4H10$) and methyl propane ($C_4H10$) are structural isomers, which have same molecular formula but different bonding patterns of atoms.

Structural isomers have different physical and chemical properties. For example, Methyl propane being a branched isomer has lower boiling point as branching leads to less surface area and thus lesser vanderwaal forces .The chain isomer Butane has higher boiling point as it has more surface area and high vanderwaal forces.