Find the density (in g/L) of NO_2(g) and the number of molecules per liter (a) a

Question

Find the density (in g/L) of NO_2(g) and the number of molecules per liter (a) at STP and (b) at room conditions (20 degree C and 1.00 atm) In a study of O_2 uptake by muscle at high altitude, a physiologist prepares an atmosphere consisting of 79 mole% N_2, 17 mole%^16 O_2, and 4.0 mole%^18 O_2. (The isotope^18O will be measured to determine the O_2 uptake.) The pressure of the mixture is 0.75 atm to simulate high altitude. Calculate the mole fraction and partial pressure of N_2 and^18O_2 in the mixture. Determine what gases move the fastest.

Explanation / Answer

(4)

(a)

* Absolute temperature at STP = 273.15 K

* Pressure at STP = 1 atm

* Molar mass of NO2 = 1(28)+2(16) = 60 g/mol

* density of NO2 at STP,

d = M P / R T

d = 60 * 1.00 / 0.0821 * 273.15

d = 2.76 g/L

(b) Absolute temperature = 273.15 + 20 = 293.15 K

Density at 20 degree C and 1.00 atm

d = M P / R T
d = 60 * 1 / (0.0821 * 293.15)

d = 2.49 g /L

(5)

Moles fraction of N2 = moles of N2 / total number of moles

XN2 = 79 / (79+17+4) = 0.79

Mole fraction of 18O2 = 4.0 / 100 = 0.040

Partial pressure of N2 = total pressure * mole fraction of N2

PN2 = 0.75 * 0.79

PN2 = 0.59 atm

Partial pressure of 18O2 = total pressure * mole fraction of 18O2

P18O2 = 0.75 * 0.040

P18O2 = 0.030 atm

(6) H2 moves faster because rate of diffusion is inversely proportional to molar mass

CO moves faster at 150 degree C because rate of diffusion is directly proportional to temperature

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