Why is the vapor pressure of water important in an experiment to determine the molar mass of Mg?

Answer 1

I don't know what specific experiment you're referring to, but in general, magnesium in water is Mg2+, and it binds water very tightly, so it reduces the vapor pressure of water (it also increases the boiling point). The more magnesium, the greater the change in vapor pressure and boiling point, and it's related to the molarity. Therefore, if you know the mass of Mg you put in, the changes in vapor pressure and boiling point will give you the molarity, and you divide one by the other to get the molar mass.