Measure the volume of your room, determine the temperature of your room, assume the pressure is one atmosphere, and using the ideal gas law calculate the moles of air inthe room. Determine how many moles of oxygen is in the moles of air and contvert the moles of oxygen to mass of oxygen. Remember the length times width times height equals the volume, and room temperature must be converted from fahrenheit to celcius to kelvin before plugging it into the equation.