A sealed flask contains 0.80 g of helium gas at 25°C and 1.2 atm. What is the volume of the flask in liters? (Molar mass of He = 4.0 g/mol; R = 0.0821 L·atm·mol⁻¹·K⁻¹)
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Correct answer
B. 4.1 L
Principle or equation
The ideal gas equation is PV = nRT, where n is the amount of substance in moles. For a fixed amount of gas, the volume can be calculated by rearranging to V = nRT/P.
Why this answer is correct
First, convert the mass of helium to moles: n = 0.80 g / 4.0 g/mol = 0.20 mol. Convert temperature to kelvin: T = 25 + 273 = 298 K. Then use the ideal gas equation: V = nRT/P = (0.20 mol)(0.0821 L·atm·mol⁻¹·K⁻¹)(298 K) / 1.2 atm = 4.08 L, which rounds to 4.1 L.
Example
For 0.50 mol of an ideal gas at 300 K and 1.0 atm, V = (0.50)(0.0821)(300)/1.0 = 12.3 L.
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