If a 0.500 l sample of helium has a pressure of 170 mm hg and a temperature of 25.0 °c, how many moles of helium are present?

So I believe that you are going to be using the formula PV = nRT and you will be solving for moles. You have everything you need for this formula: 
P = 170 mm hg **dont forget to change into atm 
V = .500 L
R = gas constant (8.314 J / mol) 
T = 25.0 C + 273 =  temperature in Kelvin 
Now it is just plug and chug 
I hope this helped! 
Have a fantastical day, 
Em