If a sample of Helium gas is collected over water, what is the pressure of the dry gas alone if the total pressure is 760.0 mmHg and the partial pressure of water is 20.0 mmHg?

Dalton's Law of Partial Pressure says that the total pressure is the sum of the partial pressure of each gas. Ptotal = PHe + PH2O

To find the pressure of the dry gas alone, we need to subtract the partial pressure of water from the total pressure.

Step 1: Total pressure - Partial pressure of water = Pressure of dry gas alone

Given:
Total pressure = 760.0 mmHg
Partial pressure of water = 20.0 mmHg

Step 2: Substitute the given values into the equation.

760.0 mmHg - 20.0 mmHg = Pressure of dry gas alone

Step 3: Perform the subtraction.

740.0 mmHg = Pressure of dry gas alone

Therefore, the pressure of the dry gas alone is 740.0 mmHg.