Molar volume is a property of a component in a solution. It is defined as the volume occupied by one mole of the component in the closed system. You would not expect all solutions to execute volume additivity because intermolecular forces between the components come into play. There is no such thing as conservation of volume.
Vapor pressure affects molar volume because gases are very sensitive by these process conditions. Vapor pressure is very temperature-dependent. Consequently, at a different temperature, your component could expand or compress, thus, affecting the molar volume. Moreover, the pressure affects the molecular collisions in the system.
First identify which is being oxidized and reduced. In this case, the Mg is being oxidized and the Hg is being reduced.
Mg --> Mg+2
<span>Hg+2 --> Hg+1
</span>
Then you have to balance each half reaction first with electrons before adding them together in one equation

⇒

and

⇒
and then combine them together to form

⇒

It isn't necessary to keep the electrons but its essential to know how many there are in order to know how many are in the equation in order to calculate the reaction energy. Note: A<span>dd H+ and H2O to balance the H's and O's in acidic solution if needed.</span>