At stp (standard temperature and pressure), the temperature is T=0 C=273 K and the pressure is p=1.00 atm. So we can use the ideal gas law to find the number of moles of helium:

where p is the pressure (1.00 atm), V the volume (20.0 L), n the number of moles, T the temperature (273 K) and

the gas constant. Using the numbers and re-arranging the formula, we can calculate n:
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