The standard state Gibbs free energies of formation of C(graphite) and C(diamond) at T = 298 K are

${\Delta }_{f}G°\left[C\left(\text{graphite}\right)\right]=0kJmo{l}^{-1}$

${\Delta }_{f}G°\left[C\left(\text{diamonda}\right)\right]=2.9kJmo{l}^{-1}$

The standard state means that the pressure should be 1 bar, and substance should be pure at a given temperature. The conversion of graphite [C(graphite) to diamond [C (diamond)] reduces its volume by $2\times {10}^{-6}{m}^{3}mo{l}^{-1}$. If C(graphite)  is converted to C(diamond) isothermally at T = 298 K , the pressure at which C(graphite) is in equilibrium with C(diamond), is [Useful information: $1J=1kg{m}^2{s}^{-2}$ ; $1Pa=1kg{m}^{-1}{s}^{-2}$ ; $1bar={10}^{5}Pa$]