?

ACT® Science

Free Version

Upgrade subject to access all content

Moderate

Explain Experiment 3

ACTSCI-PTK1BY

A student is conducting an experiment to determine what factors affect the vapor pressure of methanol. The student is measuring the vapor pressure of methanol using a mercury manometer, as pictured below. The experimental apparatus has an evacuated 1.0 liter bulb, into which the student injects a measured amount of methanol.

The methanol then vaporizes to a gas until the vapor pressure equilibrates with the liquid, by saturating the bulb volume with methanol vapor. The temperature of the methanol is controlled by placing it in a water bath. The student measures the vapor pressure by determining the height of the mercury column after the methanol is injected.

Experiment 1 Results
The student first tried varying the volume of methanol injected into the bulb.

Experiment 2 Results
Next, the student tried varying the temperature of a fixed volume of methanol. The student made sure there was always methanol liquid in the glass bulb when the measurement of vapor pressure was taken.

Experiment 3 Results
Finally, the student tried measuring the vapor pressure of different substances, both with and without alcohol (-OH) groups, at 20° C.

Which of the following BEST describes the results from Experiment 3?

A

The heavier the molecule, the longer the carbon chain.

B

Adding an alcohol to the molecule offsets the longer carbon chain and minimizes the change to the vapor pressure.

C

The molar mass is the most important factor in determining the vapor pressure of a substance.

D

Lengthening the carbon chain or adding an alcohol to the molecule both reduce the vapor pressure.