Gas Law - Determining the Value of R: Pre-Laboratory assignment Name Instructor

Question

Gas Law - Determining the Value of R: Pre-Laboratory assignment Name Instructor Date Section 1. The density of air at STP is 1,2754 gL. If your 3. Estimate the average molar mass o assuming it contains 80 % N2 and 20 % "250 mL"flask held 272 mL of water (also the volume of air it would contain), calculate the mass of air in the flask under STP. 4. Calculate the R value based on density and the average molar mass of air a. under STP 2. What would be the density of air under 20°C and 720 mmHg?(Hint: gas density is proportional to P and inversely proportional to T, see eq. 2.) b. under 20°C and 720 mmHg -63-

Explanation / Answer

Q1

mass of air in

V = 250 mL flas --> 272 mL = 0.272 Liter

mass = D*V = 1.2754*0.272 = 0.3469088 g

Q2.

Apply Ideal Gas Law,

PV = nRT

where

P = absolute pressure; V = total volume of gas

n = moles of gas

T = absolute Temperature; R = ideal gas constant

D = mass / volume and Molar volume = n/V

MW = molar weight of sample

Then,

PV = nRT; get molar volume

n/V = P/(RT)

get mass -> n = mass/MW

mass/(MW*V) = P/(RT)

D = mass/V

D/MW = P/(RT)

D = P*MW/(RT)

D = (720)(29)/(62.4*(20+273))

D = 1.142 g/L

Q3

MWavg = x1*MW1 +x2*MW2

MWavg = 0.8*28 + 0.2*32 = 28.8 g/mol

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