Chemical equilibrium

[curiosity colour]

I have a question about the solution (a) in this example, how does it know P_H2S is the same as ammonia? is there something I've miss?

 

[DrDu]

Before adding H2S, the partial pressures of H2S and NH3 are equal because of the stoichiometry of the dissociation reaction.

 

[curiosity colour]

Before adding H2S, the partial pressures of H2S and NH3 are equal because of the stoichiometry of the dissociation reaction.

Oh yeah, I totally forgot about that. Thanks for your help :D