Idon't understand this part? plz =)

What is the value of the standard enthalpy of formation of an element in its most stable form?

How many grams of sodium chloride are in 250ml of a 2.5m naci solution

Equal numbers of moles of he(g), ar(g), and ne(g) are placed in a glass vessel at room temperature. if the vessel has a pinhole-sized leak, which of the following will be true regarding the relative values of the partial pressures of the gases remaining in the vessel after some of the gas mixture has effused?