At an atmospheric pressure of 870 mm hg of 21% oxygen, the partial pressure of oxygen is

Atmosphere is made of a mixture of gases. Each individual gas exerts a pressure known as the partial pressure.
The sum of the partial pressures exerted by each gas in the mixture is equal to the total pressure exerted by the mixture of gases as a whole.
The partial pressure depends on the composition of the gas in the mixture.
In this case the total pressure - 870 mmHg
21% of atmosphere is oxygen
partial pressure of Oxygen = 870 mmHg x 21%  = 182.7 mmHg

You would be correct, boiling water does disinfect contamination.    the property of disinfecting means to kill  germs, bacteria and other pathogens or contamination  that might cause an infection or cause a person to become sickly. a couple methods of disinfecting are using soap, other cleaning supplies or by simply "boiling" the contaminate.  the reason why boiling water disinfects certain attributes is because some cells cannot survive the heat and pressure.