Calculate the number of atoms and volume of 1 gallon of helium gas at stp.

Hello!
STP means standard temperature and pressure. Denote the temperature as T (in Kelvin) and the pressure as P. They may be different for different standards.
The formula we need is  PV = nRT, where V is the volume of a gas, n is the number of moles and R is the ideal gas constant, about 8.3 J/(mol*K). The volume is also given, it is 1 gallon approx 3.8*10^(-3) m^3.
From all this data we can find the quantity of helium in moles:  n = (PV)/(RT). The number of atoms in each mole is a constant called Avogadro's constant, N_A approx6*10^(23) (mol)^(-1), so the number of atoms is n*N_A = (PV)/(RT)*N_A.
Let T = 273 K and P = 10^5 Pa, then the number of atoms is about
(10^5*3.8*10^(-3)*6*10^(23)) / (8.3*273) approx 0.01*10^(25) = 10^23.
This is the answer.