Two dimensional (2D) MXenes, denoted by a general formula M_n+1X_nT_x (n = 1−3) in which M represents an early transition metal (Ti, Cr, Zr, Nb, etc.), X is C and/or N and T stands for surface functional groups (-F, -OH, -O), are interesting candidates for a wide range of applications. They are synthesized by chemical exfoliation of MAX (M_n+1AX_n) phases (A denotes group-A element, i.e., Al, Ga, etc.) in an etchant solution, e.g., HF solution. Conventionally, the approaches based on density functional theory (DFT), are used for predicting MXenes rely on thermodynamic stability analysis, which do not consider an etchant environment. One such approach predicts the possibility of synthesizing both Ti₂C and Cr₂C MXenes for example. However, despite numerous experimental attempts, to date Cr₂C MXene has not been realized in HF solution, while Ti₂C has been synthesized. This demands for a better approach of predicting the possibility of synthesizing MXenes. In this study, we have developed a descriptor that can characterize the feasibility of exfoliation of MXenes from a parent MAX precursor by explicitly considering the etching environment. This descriptor is based on comparison of reaction energies ($\Delta_r G$) of MXenes and other competing phases (e.g., binary fluorides, and carbides) as a function of HF chemical potential. These energies are calculated by ab-initio equilibrium thermodynamics based on DFT and experimental thermodynamic data. It is found that Cr-based MXenes do not have a feasible window in which $\Delta_r G$ is negative and (absolute) higher than that of competing phases. On the contrary, Ti-based MXenes have a feasible window. Hence, we suggest that HF is not an appropriate etchant for exfoliation of Cr-based MAX phases.