Interpretation: The number of oxygen molecules in a 75.0 g sample of air needs to be determined.
Concept Introduction: The number of moles of a substance is related to mass and molar mass as follows:
Here, m is mass and M is the molar mass of the substance.
Also, the number of moles is related to the pressure of the gas as follows:
Here, P is pressure, V is volume, n is the number of moles, R is the Universal gas constant and T is temperature.
Explanation of Solution
The percentage of oxygen by volume in dry air is given 20.95%.
Thus, the relation between the volume of oxygen and air is:
The mass of the air is given 75 g and the density of air is 1.19 g/L. thus, the volume of air can be calculated as follows:
Substitute the values,
Now, the volume of oxygen in the air will be:
Therefore, the volume of oxygen gas is 13.20 L.
Now, at STP the value of temperature is 273.15 and pressure is 1 atm. Thus, the number of moles of oxygen gas can be calculated as follows:
Or,
Substitute the values,
Therefore, the number of moles of oxygen gas is 0.5893 mol.
Now, the mass of oxygen gas can be calculated as follows:
Molar mass of oxygen gas is 32 g/mol.
Thus, the mass of oxygen gas is 18.86 g.
Chapter 10 Solutions
Chemistry 2012 Student Edition (hard Cover) Grade 11
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