The experiment you will determine the molar mass of carbon dioxide by measuring

Question

The experiment you will determine the molar mass of carbon dioxide by measuring the mass of an Erlenmeyer flask full of the gas. The following calculations are intended to familiarize you with the general procedure: The mass of an empty Erlenmeyer flask and stopper was determined to be 54.93 grams. When filled with distilled water, the mass was 296.8 grams. The atmospheric pressure was measured to be 0.9157 atm, the room temperature was 25.00oC. At this temperature, the vapor pressure of water is 23.80 torr -- but assume a 50% relative humidity as outlined in the procedure for this experiment. 1.)What is the number of moles of air in the flask? Moles of air in the flask = _______mol 2.)If the average molar mass of the gases present in air is 28.960 g mol-1, what is the mass of air in the flask? Mass of air in the flask = __________grams 3.)What would be the mass of carbon dioxide in the flask? Mass of carbon dioxide = ________grams 4.)What would be the mass of the flask (and stopper) when filled with carbon dioxide gas? In this case the CO2 will be saturated with water vapor so assume 100% humidity as outlined in the experiment. Mass of flask and stopper filled with CO2 gas = _______grams

Explanation / Answer

clues...
(1) you have mass of empty flask + mass stopper AND you have mass of water + mass flask + mass stopper. how can you use that data?
(2) what do you suppose the relationship is between relative humidity and saturated vapor pressure?
(3) can you apply Daltons law of partial pressures here?
(4) gas laws should make you think PV = nRT
ok?

now.. that you've thought about those, read on.
let's do these steps...
(1) calculate volume of the flask.. = mass water x 1/density of water
(2) calculate PH2O.. via relative humidity x saturated vapor pressure
(3) via Daltons law of partial pressures Ptotal = Pair + PH2O
(4) PV = nRT.. n = PV/(RT).. n-air = Pair x Vflask / (R x T)
(5) convert moles air to mass air
(6) that "n" in PV = nRT doesn't depend on which gas is present. So n-air = n-CO2
(7) convert nCO2 to mass CO2
(8) and that last part is a bit tricky. We have to start over with PH2O = 16.50torr and calculate PCO2 and nCO2 and mass CO2 and mass H2O

make sense?

solutions...
*** 1 ***
mass water = (305.4g - 47.92g) = 257.48g
density of water = 0.9983 g/mL @19

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