If a 0.500 L sample of helium has a pressure of 170 mm Hg and a temperature of 25.0 and degC how many moles of helium are present?

To find the number of moles of helium, we can use the ideal gas law, ( PV = nRT ). First, convert the temperature to Kelvin (25.0 °C = 298.15 K) and the pressure to atmospheres (170 mm Hg = 0.223 atm). Using the gas constant ( R = 0.0821 , \text{L} \cdot \text{atm} / (\text{mol} \cdot \text{K}) ), we rearrange the equation to ( n = \frac{PV}{RT} ). Substituting the values: ( n = \frac{(0.223 , \text{atm})(0.500 , \text{L})}{(0.0821 , \text{L} \cdot \text{atm} / (\text{mol} \cdot \text{K}))(298.15 , \text{K})} \approx 0.0044 , \text{moles} ).