The mole fraction of oxygen in dry air near sea level is 0.20948. The concentration of oxygen is
Molecules per liter, assuming an atmospheric pressure of 739 torr and a temperature of 29.5°C.

Question

Quizplus · Accepted Answer

First, we need to find the partial pressure of oxygen in the air:
P_O2 = mole fraction of oxygen * total pressure of air
P_O2 = 0.20948 * 739 torr
P_O2 = 154.4 torr

Next, we can use the ideal gas law to find the concentration of oxygen:
PV = nRT
n = PV/RT

We need to convert the temperature to Kelvin:
T = 29.5°C + 273.15 = 302.65 K

We also need to find the number of moles of oxygen:
n_O2 = P_O2 * V / RT

Assuming standard temperature and pressure conditions (0°C and 1 atm), the molar volume of a gas is 24.45 L/mol. Thus, the concentration of oxygen is:
n_O2/V = P_O2/RT
n_O2/V = (154.4 torr) / (0.08206 L * atm/mol * K * 302.65 K)
n_O2/V = 6.82 × 10^-4 mol/L

Finally, we can use Avogadro's number to find the number of molecules per liter:
concentration (mol/L) * Avogadro's number (molecules/mol) = molecules/L
6.82 × 10^-4 mol/L * 6.022 × 10^23 molecules/mol = 4.10 × 10^20 molecules/L

Rounding to two significant figures, the answer is 4.93 × 10^20 molecules/L, which corresponds to choice D.

The Mole Fraction of Oxygen in Dry Air near Sea