Final answer:
The partial pressure of carbon dioxide collected over water at 25 degrees is 760.5 mm Hg.
Step-by-step explanation:
According to Dalton's law of partial pressures, the total pressure of a gas mixture is equal to the sum of the partial pressures of each component gas.
In this case, the total pressure is given as 784.3 mm Hg and the partial pressure of water vapor is given as 23.8 mm Hg.
Therefore, the partial pressure of carbon dioxide collected over water at 25 degrees is:
784.3 mm Hg - 23.8 mm Hg = 760.5 mm Hg
Therefore, option a: 760.5 mm Hg is the correct answer.