In 78.41 liters of neon gas at STP, you have 3.498 moles of the gas.
Use the ideal gas law to calculate the value of moles.
The ideal gas law is PV=nRT
P is pressure which is 1 atmospheres for standard conditions.
V is volume which in this problem is given as 78.41 liters.
n is the mole value which we are trying to obtain.
R is the universal gas constant which is
0.0821 liters x atmospheres/moles x kelvin
T is the temperature in Kelvin which 273 K for standard conditions.
Rearrange the equation to solve for n
n = PV/RT
n = ( 1 atm) ( 78.41 L)/[(0.0821 L atm/mol K)(273)]
n= 3.498 moles of Neon gas