Thermodynamics tables and charts may be used when both H and S are tabulated. Since G = H - TS, at constant temperature, ΔG = RT ln(f₂/f₁) = ΔH - TΔS. If state 1 is at low pressure where the gas is ideal, then f₁ = P₁, RT ln(f₂/P₁) = ΔH - TΔS, where the subscripts indicate states. Use this method to determine the fugacity of steam at 400 °C and 15 MPa. What value does the fugacity coefficient have at this pressure?