What is an isomer? how many possible isomers of hexane are there? what are the structural differences between these isomers?

An isomer is a compound that differs by another based on either the molecular connectivity or how the atoms are connected to each other in the compound, or by stereochemistry usually.

I believe there are total number of 5 isomers for the compound of C6H14. Hexane.

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.