In order to determine the number of particles of methane, use the following expression:
number of particles = (mass /molar mass ) x Avogadro's number
in this case, you have:
molar mass = 16.03 grams/mole
mass = 120 grams
Avogadro's number = 6.02*10²³ particles/mol
replace the previos values of the parameters into the formula for the number of particles:
number of particles=(120 grams/ 16.03 grams/ mol)x(6.02*10²³particles/mol)
= (7.46 mol) x (6.02*10²³ particles/mol)
= 4.50*10²⁴ particles
Hence, the number of particles of methane, in 120 grams, is 4.50*10²⁴