When hydrogen gas is collected by displacement of water in a pneumatic trough
CHEMISTRY · Single correct · A Level · Chemistry · Amount of substance · Amount of substance
Problem
When hydrogen gas is collected by displacement of water in a pneumatic trough, which statement best explains why the vapor pressure of water must be subtracted from the barometric pressure before using the ideal gas law to determine the amount of hydrogen?
(a) Because the vapor pressure of water pushes inward opposite to the hydrogen, it subtracts from the total, so the correction must be made to find the true hydrogen pressure.
(b) Water vapor contributes to the total pressure inside the collection vessel, so only the remaining pressure is due to hydrogen.
(c) Subtracting the vapor pressure corrects for the weight of the water column above the gas, which is included in the barometric reading.
(d) Hydrogen dissolves partially in water, lowering its pressure by the amount equal to the vapor pressure, so this value must be subtracted.