A correlation between conductance change and corresponding surface work function (SWF) change due to molecular adsorption on epitaxial graphene grown on both C and Si-faces of 6H-SiC has been demonstrated based on Fermi level shifts, type of carrier, and density of states D(E) of the graphene film. Graphene samples were grown epitaxially on CMP polished Si and C-faces of semi-insulating on-axis 6H-SiC substrates by annealing them at 1300-1600<SUP>O</SUP>C for one hour under high vacuum (<10<SUP>-5</SUP> Torr) in a cold-wall resistively heated SiC sublimation reactor. From Raman spectroscopy, thickness of Si-face graphene was found to be 1~3 MLs, while that of C-face was 8-10 MLs. The low I<SUB>D</SUB>/I<SUB>G</SUB> (‘D’ peak to ‘G’ peak) ratio (<0.02) confirmed quality graphene. In amperometric experiments, p-type doping with electron accepting NO<SUB>2</SUB> caused the conductance to decrease by 9% for Si-face few layer graphene (FLG) and to increase by 3.25% for C-face multilayer graphene (MLG). Adsorption of electron donating NH<SUB>3</SUB> increased the conductance of Si-face FLG by 8% and decreased that of C-face MLG by 3.5%. Therefore, p-type (n-type) doping by NO<SUB>2</SUB> (NH<SUB>3</SUB>) should decrease (increase) the conductivity of an n-type graphene layer, and increase (decrease) the conductivity of p-type graphene layer. As per our observation, this infers that in C-face MLG p-type charge carriers are dominant and in Si-face FLG charge carriers are mostly n-type. Lin et al. supports our observation. To support our amperometric results, and to examine in more detail, we performed the potentiometric experiments, and found a strong correlation between them. Adsorption of NO<SUB>2</SUB> (NH<SUB>3</SUB>) caused the SWF of graphene on both the faces to increase (decrease) by 120~140 meV (85~100 meV) implying a downward (an upward) shift of the Fermi level by that amount. If the Fermi level of Si-face FLG, E<SUB>F-Si</SUB>, goes down, D(E) for n-type carriers decreases, suggesting a decrease in the conductance. Again, if the Fermi level of C-face MLG, E<SUB>F-C</SUB>, goes down, D(E) for p-type carriers increases implying an increase in the conductance. If NH<SUB>3</SUB> adsorption pulls E<SUB>F-Si</SUB> up, D(E) for n-type carriers increases which causes the conductance of Si-face FLG to increase. Again if E<SUB>F-C</SUB> is pulled up, D(E) for p-type carriers decreases resulting in a decrease in the conductance of C-face MLG. Flow rate dependence of SWF change of an adsorption system has also been observed for Si-face FLG. The observed differences in SWF changes can be explained by considering the change in electron affinity of 6H-SiC due to the formation of a dipole moment between the polar molecules (NO<SUB>2</SUB>, NH<SUB>3</SUB>) and the substrate inferring that at higher flow pressure gas molecules can diffuse through the graphene film to the substrate.