If I have Iron (nail) and 0.5 mol of Hydrochloric acid in a solution of water, will the nail loose weight?

The nail must lose m, because some of the Fe becomes Fe++ ions, which p into solution. The leftover electrons from the iron bind with H+ ions from the acid to give hydrogen gas, so the system of (nail + solution) has a net loss in m. However, the solution itself will have *gained* m, because a single Fe++ ion is heavier than two H+ ions.