The equilibrium coverage of metal electrodes in contact with an electrolyte has been addressed by periodic quantum-chemical calculations based on density functional theory. The electrolyte has been treated in a grand-canonical approach using the concept of the computational hydrogen electrode. After briefly reviewing the theory and illustrating it using the coadsorption of bromide and hydrogen as an example, the interaction of nitrate with a Pb(111) electrode is addressed. A spontaneous reconstruction of the uppermost Pb layer is observed leading to the formation of a salt-like layer.