A sample of He collected over water at a temperature of 20C has a pressure of 705 torr. What is the pressure of the dry gas? The vapor pressure of water at 20 degrees * C is 17.5 torr.

The pressure of the dry gas : 687.5 torr

Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases  

Can be formulated:  

P tot = P1 + P2 + P3 ....  

The partial pressure is the pressure of each gas in a mixture  

Since the gas mixture consists of He and H₂O :

P tot=P He + P H₂O

plants need sunlight, water, and carbon dioxide to make food. plants use the raw materials of water, nutrients they absorb from soil, and carbon dioxide to make sugars the plant can use. energy from the sun and chlorophyll are also needed.