The atomic mass of carbon -13 is 13 it has six protons how many neutrons does this isotope have

It has 7 neutrons. The atomic mass is the sum of protons and neurons, therefore you subtract the number of protons from the atomic mass to get the number of neutrons.

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.