Lab 4

How Can We Identify an Unknown Gas?

Unit 4⏱ 90 minGuided Inquiry

Overview

How can we determine the molar mass of a substance from measurements of the density of its vapor?

Objectives

How can we determine the molar mass of a substance from measurements of the density of its vapor?
Connect observations to AP Chemistry models and calculations.

⚠️ Wear protective goggles and an apron. Follow your teacher’s safety directions for all chemicals and equipment.

Materials Needed

unknown liquid safety glasses Parafilm balance aluminum foil (5-cm square) 600-mL beaker utility clamp pin -mL graduated cylinder paper towels lab apron or coat hot plate -mL Erlenmeyer flask ring stand crucible tongs -mL graduated cylinder boiling chips Tie back long hair and secure loose clothing when working with an open flame Always wear safety goggles and a lab apron or coat when working in the lab

Procedure

Set up a ring stand and hot plate for the purpose of heating (as demonstrated by your teacher).

Place a 600-mL beaker about ½ filled with water and boiling chips on the hot plate.

Heat the water to boiling.

While the water is heating, complete steps 2, 3, and 4.

Find the mass of an empty 125-mL Erlenmeyer flask.

Record this mass as (a) in your data table.

Using a 10-mL graduated cylinder, add 3 mL of the unknown liquid to the flask.

Using a 5-cm square piece of aluminum foil, make an air-tight cap over the mouth of the flask.

Cover the foil with some Parafilm.

With a pin, make a “very tiny” hole (as small as possible) in the center of the cap.

Once the water is vigorously boiling, carefully lower the flask into boiling water, and clamp the flask securely to the ring stand.

Make sure that at least ½ of the flask is submerged in the water, but that the cap is not.

Pre-Lab Questions

1. Answer the objective and mention what the unknown was and how you determined it. What was the molar mass of your unknown? Discuss any sources of error and explain WHY you believe this would cause you to either get a larger or smaller molar mass compared to the actual molar mass. (Note your explanation must be in line with your result).

Post-Lab Questions

1. NOTE: when handing in the formal lab, you must write down the questions as well as the answers. They must ALWAYS be in complete sentences. What are some possible sources of error for this lab? A gas has a density of 1.25 g/L at STP. What is its molar mass? What could be the identity of this gas? AT STP, 10 liters of a gas has a mass of 9.01 grams. What is the molar mass of this gas? What could be the identity of this gas? What is the Van der Waals equation that accounts for the inaccuracies of the ideal gas equation? What do ALL of the symbols stand for? What is the density of CO2 gas at 750 torr and 20oC? How many molecules of CO2 gas would be produced from the complete combustion of 200 mL of propane gas at 725 torr and 35oC? (Note you must write out the balanced reaction to determine this answer).