Vapor Pressure: Water and Ethanol
Barometers are shown that read 736 mmHg. Water is injected into one and ethanol is injected into the other, and the vapor pressure of each liquid is determined. Molecular models of water and ethanol are shown and hydrogen bonding is illustrated for each molecule. The 18 thumbnail images summarize the content of the video. Click an image to see the image gallery.
The height of the mercury in each tube is 736 millimeters. A syringe is filled with water and carefully injected into the open end of the barometer (by going underneath the pool of mercury.) The liquid rises up to the top of the mercury column. When it reaches the vacuum at the top of the barometer, some of the liquid will vaporize. This depresses the mercury column to 716 millimeters. (The difference in the height of the mercury column before and after injecting the liquid is the vapor pressure of the liquid.) The vapor pressure of water is 20 millimeters of mercury.
A single water molecule can form hydrogen bonds between itself and up to four other water molecules. Vapor pressure varies with the strength of the intermolecular forces in the liquid. Ethanol can only form up to three hydrogen bonds, and they are not as strong as the hydrogen bonds in water. When ethanol is injected into a barometer, (the ethanol rises to the top and vaporizes and) the mercury is depressed more than it was with water. More ethanol has vaporized indicating that it has weaker intermolecular forces than water.
Barometers can be used to measure vapor pressures of liquids and to demonstrate relative vapor pressures of different liquids. Vapor pressures of water and ethanol are compared, and they are found to vary with the strength of intermolecular forces. Water has more hydrogen-bonding opportunities and forms stronger hydrogen bonds than ethanol, and the vapor pressure of water is found to be less than for ethanol.
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