Although electrons have mass, they are not considered in determining the mass number of an atom. why?

If i initially have a gas at a pressure of 12 atm, a volume of 23 liters, and a temperature of 200 k, and then i raise the pressure to 14 atm and increase the temperature to 300 k, what is the new volume of the gas?

What substance has a mass of 9.5g and volume of 2.1cm^3

What mass of natural gas (ch4) must you burn to emit 276 kj of heat?