Answer
(a) There are 3 triple points in the phase diagram.
(b) Under atmospheric conditions, the rhombic allotrope is more stable.
(c) At 1 atm and $80^{\circ}C$, Sulfur exists as a rhombic allotrope as it is stable in these conditions. When the temperature crosses $95.4^{\circ}C$, rhombic sulfur gets converted to monoclinic sulfur as it is stable in these conditions and as the temperature reaches $119^{\circ}C$, the solid monoclinic sulfur starts melting; as the temperature is increased further, all of the monoclinic sulfur gets converted into liquid sulfur.
Work Step by Step
(a) There are 3 triple points in the phase diagram.
(b) Under atmospheric conditions, the rhombic allotrope is more stable.
(c) At 1 atm and $80^{\circ}C$, Sulfur exists as a rhombic allotrope as it is stable in these conditions. When the temperature crosses $95.4^{\circ}C$, rhombic sulfur gets converted to monoclinic sulfur as it is stable in these conditions and as the temperature reaches $119^{\circ}C$, the solid monoclinic sulfur starts melting; as the temperature is increased further, all of the monoclinic sulfur gets converted into liquid sulfur.
