[stellarox]

Hi, i would like to clear my doubt about my recent examination questions on chemical equilibria. Here we  go...

Steam dissociates into its elements at high temperature.

2H₂O (g) <->  2H₂ (g) + O₂ (g)

The reaction is carried out at a very high temperature and 1atm.

At equilibrium, partial pressure of H₂ is found to be 0.200 atm .

Calculate the partial pressure of oxygen and steam at equilibrium.

My ans was...

Partial pressure for oxygen = 0.200/2
                                        = 0.100 (correct)

Partial pressure for steam = 0.200 (wrong)

i don't understand why it is wrong because i thought according to the mole ratio, the partial pressure for steam should be equal to the partial pressure of hydrogen. Perhaps my concept is wrong, so can someone enlighten me? Thanks!

(paraphrased by geodome. please refrain from using internet shorthand in your future posts)

[Donaldson Tan]

2H₂O (g) <->  2H₂ (g) + O₂ (g)

the stoichiometric ratio tells you that for every 2 mole of steam dissociated, 2 moles of hydrogen gas and 1 mole of oxygen is gas is produced.

assuming isobaric conditions, ie. steam is heated to a very high temperature at 1atm.

Partial pressure for oxygen = 0.200/2
                                        = 0.100 (correct)

all oxygen and hydrogen present comes from the dissociation of steam and there was zero amount of oxygen and hydrogen initially. Hence, the amount of hydrogen and oxygen present at equilibrium must follow the stoichiometric ratio.

Partial pressure for steam = 0.200 (wrong)

this is wrong because the amount of steam at equilibrium does not follow stoichiometric ratio. There was an intial non-zero amount of steam. the amount of steam reacted agrees with the stoichiometric ratio, but not the amount remained. In fact, we know the partial pressure of hydrogen and oxygen at equilibrium. Using Dalton's Law of Partial Pressure, the partial pressure of steam therefore is 1.000 - 0.200 - 0.100 = 0.700atm