Lab #5 Molar Mass Using PV = Nrtname

Lab #5–Molar Mass using PV = nRTName

Background

In order to determine an unknown compound’s molar mass, one must be able to determine the number of moles in the unknown. If the substance is a gas, or an easily vaporized liquid, this can be done with relative ease using the Ideal Gas Law, PV = nRT, where n is the number of moles. By determining the Pressure of the gas, its Volume, and its Temperature, we can calculate n = PV/(RT).

In this procedure we will introduce a small sample of a volatile liquid into a specialized piece of glassware called a Dumas flask (MSRP $20). The liquid will be completely vaporized by heating in a water bath so that the flask contains only vapor. The temperature of the water bath is taken to be the same as the temperature inside the flask. Since the flask is an open container, the pressure inside the flask is the same as atmospheric pressure. Determining the volume inside the flask is problematic and will be discussed in class.

Procedure

Fill a 1000 mL beaker to within 4 finger widths of the top edge. Heat the water to boiling.

While waiting for the water bath to heat, use KimWipes to wipe down a Dumas flask. Use the analytic balance to find the mass of the clean Dumas flask w/stopper. Record the mass of the empty flask w/stopper. Also find and record the mass of the empty Dumas flask w/out the stopper.

Use the technique discussed in class to introduce about 5 mL of the unknown liquid into the Dumas flask. Once the water bath is boiling, use the inverted clay triangle/ring apparatus to submerge the unstoppered Dumas flask in the water bath. Be careful not to get water in the Dumas’ spout. While the flask is being heated, avoid creating air currents around the mouth of the Dumas flask, as this can create a pressure difference that might draw vapor out of the flask. Continue heating until no more liquid is visible in the Dumas flask. Record the temperature of the water bath, then turn off the burner.

Once the water has stopped boiling, stopper the flask to prevent vapor loss during cooling and remove the flask from the water bath. As the flask cools, remove the stopper briefly every 30 seconds to allow pressure to equalize as the vapor condenses. Submerge the flask in a beaker of room temp tap water for several minutes to facilitate cooling, being careful not to get water in the spout.

Once the flask is cooled to room temp, wipe the outside with KimWipes and weigh the flask w/stopper and remaining sample on the analytic balance. Record the mass.

Dispose of the leftover volatile liquid in the waste beaker in the fume hood. Use the same logic employed to draw liquid into the flask to expel liquid from the flask.

Use the technique discussed in class to determine the internal volume of your Dumas flask.

Prelab summary

Procedural Tips: Always wipe down any container before weighing on the analytical balance.

Be sure to record your unknown number.

Start the water bath first.

Do not get water into the Dumas flask.

Avoid air currents around the mouth of the Dumas flask

Safety: All of the unknowns are flammable. Keep away from open flame.

Disposal: Leftover volatile liquid goes in waste beaker in fume hood.

Data and CalculationsUnknown #Show calculations here:

Mass of empty Dumas w/stopper g

Mass of Dumas w/stopper and condensed sample g

Mass of condensed vapor g

Temperature of water bath ºC

Atmospheric pressure (see Blackboard) torr

*Internal volume of Dumas flask (see below) mL

Moles of condensed vapor mol

Molar Mass of vapor g/mol

Determining internal volume:

External volume of flask mlExternal volume of flask ml

Mass of flask w/out stopper gMass of flask w/out stopper g

Density of glass 2.40 g/mLDensity of glass 2.60 g/mL

Volume of glass mLVolume of glass mL

Internal volume of flaskmLInternal volume of flask mL

*Average internal volume mL

Prelab Questions

1. Suggest a procedure for getting a few mL of a liquid UK into the tiny opening of the Dumas flask. Hypodermic needles will not be available. [Hint: How can we create “suction” inside the flask?]

2. Suggest a procedure for determining the external volume of the flask. Filling the flask with water is not a practical option.

3. Suggest a way to determine the volume of glass in the flask

4. A student weighs an empty Dumas flask w/stopper and finds its mass to be 36.7925 g. The flask is heated in a 98.5ºC water bath until the flask contains only vapor. The atmospheric pressure that day is 752 torr. The flask has an internal volume of 272.4 mL. The mass of the flask and condensed vapor at the end of heating is found to be 37.7138 g.

(a) What is the pressure of the vapor in the flask during heating? Explain. torr

(b) What is the temperature of the vapor during heating? Explain. ºC

(d) Calculate the molar mass of the vapor. Show all work. g/mol

2. Would the calculated molar mass of the vapor be higher, lower, or unchanged if the following procedural errors were to occur? Explain your choice by tracing the error through the calculation of molar mass.

(a) Some unvaporized liquid remains in the flask when heating is stopped.

(b) Some vapor is drawn out of the flask by air currents near the flask’s mouth.