Some hydrogen gas is collected over water at 20.0°C. The levels of water inside and outside the gas-collection bottle are the same. The partial pressure of hydrogen is 742.5 torr. What is the barometric pressure at the time the gas is collected?

Answer : The barometric pressure at the time the gas collected is, 760 mmHg

Explanation :

According to the Dalton's Law of partial pressure, the total pressure of a  mixture of gases is equal to the sum of the partial pressures  of the component gases.

where,

= total pressure

= partial pressure of hydrogen gas = 742.5 mmHg

= partial pressure of water vapor at = 17.5 mmHg  (standard value)

Now put all the given values in the above expression, we get:

Therefore, the barometric pressure at the time the gas collected is, 760 mmHg