(C)
If the collection vessel is exposed to the surface of water, as it usually is when collecting a gas in the laboratory, then water evaporates from the surface and combines with the collected gas. Therefore, to be accurate in calculating how many moles of gas are produced in the reaction, Dalton's law must be used to account for the moles of water vapor also in the collection vessel. Dalton's law states that the total pressure exerted by a mixture of gases equals the sum of the partial pressures of each component in the mixture.