Why does methane exist as a gas and nonane exist as liquid​

Molecular weight

Explanation:

The difference in the physical state of nonane vs methane at STP.  Methane is a very small molecule with a molecular weight of 16 g/mol.  It exists as a gas at standard temperature and pressure.  Nonane is a much larger molecule with a molecular weight of 128.2 g/mol.  It exists as a liquid at standard temperature and pressure.  

All gasses at STP are Made of small molecular weight molecules.  The smaller a molecule, the more movement the individual atoms have.  The increased movement of smaller molecules allows them to have more space between individual molecules.  The increased molecular motion and space between molecules is one of the main differences between a gas versus a liquid or solid.  The larger nonane molecules have less molecular motion and so are can be closer together and so exist as a liquid instead of a gas.

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.