Question

Under SATP (not STP), magnesium was reacted with acid and 500 ml of hydrogen gas was collected over water. a) what is the vapour pressure of water? b) what is the pressure of the hydrogen gas? c) what amount of hydrogen was produced?

Answer 1

(a) `P_"H₂O" = "0.0317 bar"`; (b) `P_"H₂" = "0.9683 bar"`; (c) the reaction produced 0.0199 mol of hydrogen.

Explanation:

(a) Vapour pressure of water

SATP is defined as 25 °C (298.15 K) and 1 bar.

At 25 °C, the vapor pressure of water is 3.17 kPa = 0.0317 bar,

(b) Partial pressure of hydrogen

Per Dalton's Law of Partial Pressures,

`P_"H₂" + P_"H₂O" = P_"atm"`

`P_"H₂" = P_"atm" - P_"H₂O" = "1 bar - 0.0317 bar" = "0.9683 bar"`

(c) Amount of hydrogen

`PV = nRT`

`n= (PV)/(RT) = (0.9683 color(red)(cancel(color(black)("bar"))) × 0.500 color(red)(cancel(color(black)("L"))))/("0.083 14" color(red)(cancel(color(black)("bar·L·K"^"-1")))"mol"^"-1" × 298.15 color(red)(cancel(color(black)("K")))) = "0.0199 mol"`