The mole fraction of oxygen in dry air near sea level is 0.20948. The concentration of oxygen is ________ molecules per litre, assuming an atmospheric pressure of 0.985 bar and a temperature of 29.5 °C.

Question

Quizplus · Accepted Answer

First, we need to calculate the pressure in atm since the mole fraction is given as a ratio of partial pressures:

Pressure = 0.985 bar / 1.01325 bar/atm = 0.971 atm

Next, we need to use the ideal gas law to find the concentration of oxygen in molecules per liter:

PV = nRT

n/V = P/RT

where n/V is the concentration in moles per liter.

Rearranging the equation and plugging in the given values:

n/V = (0.20948)(0.971 atm)/(0.08206 L⋅atm/mol⋅K)(302.65 K)

n/V ≈ 0.00856 mol/L

Now we can use Avogadro's number to convert from moles to molecules:

(0.00856 mol/L)(6.022×10^23 molecules/mol) ≈ 5.14×10^21 molecules/L

Rounding to two significant figures gives:

Answer: C
 The concentration of oxygen is approximately 4.94 × 10^21 molecules per liter.

The Mole Fraction of Oxygen in Dry Air near Sea