We show how the use of appropriate variables results in a flash calculation that uses only equilibrium constraints; it is thus not necessary to solve the mass balance equations self-consistently with the equilibrium equations. We use this implicit material balance in flash calculation. We show its advantages over the current approach that uses an explicit material balance. For the flash calculation for CO2 storage in brine aquifers, use of appropriate variables also allows us to find the dew, bubble, and precipitation points where the liquid, vapor, and solid salt phases, respectively, emerge. Our calculation includes the water content of the vapor phase, which arises from evaporation of the brine. Evaporation leads to increased brine salinity, which results in a large reduction in CO2 solubility in the salting-out effect, and eventually in precipitation of solid salt and ultimately the disappearance of the liquid phase. The flash calculation also relies on our derivation of fugacities for H2O and CO2 in both the brine and the vapor phase.