A mixture of H2 and water vapor is present in a closed vessel at 20°C. The total pressure of the system is
755.0 mmHg
Partial pressure of water vapor at 20°C equals 17.5 mmHg.
What is the partial pressure of H2?

Answer is: the equilibrium  shifts toward the reactants, to the left side of chemical reaction.it is because lead(ii) nitrate dissociates in water: pb(no₃)₂ → pb²⁺(aq) + 2no₃⁻(aq).according to lechatelier's principle, the position of equilibrium moves to counteract thechange (more lead cations, common ion), the position of equilibrium will move so that the concentration ofproducts decrease and concentration of reactants of chemical reaction increase.read more on -

Burned piece of steel wool does not gain mass. so only d could be true.