What is the pressure of a 3.00 L gas vessel that has 18.0 grams of helium at 25°C? (R= 0.0821 L atm/ mol K)

This is an ideal gas law question. You need to use the equation PV=nRt. First you need to find n, the number of mols of helium. The molar mass of helium is 4.00g/mol, and 18g/4.00g/mol = 4.5 mols of helium. Next you need to convert the temperature from Celsius to Kelvin, because only kelvin temperatures can go into the deal gas law equation. To convert 25C to kelvin, add 273. That gives you 298K. Now you can plug all of you information into the ideal gas law equation and solve for P, pressure.
PV=nRt
P(3.00L)=(4.5mol)(0.0821LatmbmolK)(298K)
P=36.70atm
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