What are the molar weight of methane?

The "kilogram" was defined as the mass (not weight, but similar) of one liter of water (i.e. one cubic decimeter of water) at standard temperature and pressure. There are 1 thousand cubic decimeters in a cubic meter (or 1 million cubic centimeters), so a a cubic meter of water would have a mass of 1 million grams, or 1,000 kilograms, or 1 metric ton.

This value will change when the water is not at room temperature and normal atmospheric pressure.

This is why the kilogram definition is now related to a prototype made of iridium and platinium.