I'm confused with how the Gibbs Phase Rule works for pure substances in liquid-vapor mixtures. According to the phase rule, there should only be one degree of freedom for a two-phase mixture with a pure substance. For example, if we pick a temperature for two phases to exist, there will only be one pressure that satisfies this condition. But how is the amount of vapor in the mixture, defined?
Looking at the figure below, it appears as if the pressure and specific volume are both independent of the two-phase region. Does this contradict the phase rule? If I wanted to obtain a vessel containing a mixture of water with 50% vapor and 50% liquid, could it be done by changing the pressure and temperature alone?