Chemistry
posted by Kiesha .
Please show me the steps.....
At STP, 3.00 L of nitrogen gas contains the same number of molecules as
A) 1.00 L of nitrogen gas
B) 2.00 L of oxygen gas
C) 3.00 L of chlorine gas
D) 4.00 L of hydrogen gas

You don't need an steps. Avogadro's Law says that equal volume of (ideal) gases at the same temperature and pressure contain the same number of molecules. So you had 3.00 L of one gas, you look for the answer of 3.00 L of another gas if you are looking for the same number of molecules.
You MAY calculate if you wish, using the ideal gas law equation.
PV = nRT
For the same P, V, and T, n will be the same for any gas; therefore, the number of molecules will be the same. 
How many moles are present in 30.0 L of helium gas at STP?