Chemistry Honors: Class 57

Dates

• Period A: Monday, May 16th
• Period B: Tuesday, May 17th
• Period F: Tuesday, May 17th

Lab Prep Day – Molar Volume of a Gas

Concepts to Consider

• Dalton’s Law of Partial pressure
• gas stoichiometry
• Combined Gas Law
• Ideal Gas Law
• Molar Volume

Sample Problem

• A student collects hdrogen gas as the result of the complete reaction reaction of magnesium ribbon with concentrated hydrochloric acid. She measures the volume of the gas (at level liquids) to be 44.5 mL. The temperature of the collection water is 23°C and the vapor pressure of water at that temperature is 21.1 mmHg. The air pressure in the room is 31.3″ Hg. She used 0.0485 grams of magnesium to generate the gas
• What reaction takes place?
• What is the air pressure of the room (mm Hg)?
• What is the pressure of the gas (atm)?
• What is the molar vollume at room conditions?
• What is that molar volume translated to STP?
• What percent error does she have in her lab?

Lab Talk

The lab is an example of calcualting the volume of a gas produced by a reaction [Instructions in Schoology]. The reaction is quite simple as magnesium is reacting with hydrochloric acid, producing hydrogen gas. What if this gas was captured? Is it possible to use this as a way to determine the molar volume of a gas? Obviously the answer is “yes” as this is what we will do. There are a few aspects to keep in mind when performing this lab.

• make sure the magnesium is the limiting reactant in this process so that it can predict the moles of hydrogen gas produced
• be sure and level the tube to the same level as the water outside of the tube, guaranteeing that the pressure of the gas inside the tube is the same as that outside the tube (atmospheric pressure)
• the temperature of the water is assumed to be the temperature of the gas, which is a great assumption given that the gas was produced and then bubbled through the water.
• the partial vapor pressure of water depends only upon the temperature of the water [LINK] and is to be subtracted from the total pressure of the gas to find the pressure of just the hydrogen
• once corrected, the molar volume of the gas is a simple “combined gas law” problem.