What is a difference between a mixture of elements and a mixture of compounds

Element is a pure substance consisting of one type of atom only. Mixture is two or more different substances are mixed but they are not chemically bonded together and may be separated by a physical process such as filtration, distillation and so on.

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.