The question
Name: ______________________ Date: ____________ Molar mass of the gas in a lighter (10 points)
Data table
Collecting lighter gas over water
A student weighs a disposable lighter that contains liquid butane, C₄H₁₀. Holding the lighter under water, she releases gas into an inverted graduated cylinder that started completely full of water, until some gas has collected. She raises or lowers the cylinder until the water level inside matches the water level outside, so the gas inside is at room pressure. She dries the lighter and weighs it again. The gas in the cylinder is butane mixed with water vapor.
| Quantity | Value |
|---|---|
| Mass of lighter before | 25.432 g |
| Mass of lighter after | 25.218 g |
| Volume of gas collected | 92.0 mL |
| Water temperature | 22.0 °C |
| Barometric pressure | 748.0 torr |
| Vapor pressure of water at 22.0 °C | 19.8 torr |
(a) Calculate the partial pressure of butane in the cylinder, in torr. [1 point]
(b) Calculate the number of moles of butane collected. [2 points]
(c) Calculate the molar mass of butane from the data. [1 point]
(d) A second student did not subtract the vapor pressure of water and used 748.0 torr as the pressure of butane. Predict whether that student's molar mass is greater than, less than or equal to the value in part (c), and justify your answer. [2 points]
(e) Inside the sealed lighter, butane is a liquid with its vapor above it at about 2.2 atm at 22 °C. When the valve opens, the butane comes out as a gas. Explain both observations in terms of vapor pressure. [2 points]
(f) The accepted molar mass of butane is 58.12 g/mol. A student claims that the experimental value is high because attractions between butane molecules make the gas behave non-ideally. Justify whether attractions between molecules would make the calculated molar mass too high or too low. [2 points]