This paper deals with natural convection flows evolving inside an ended and differentially heated cavity, which is filled either with an air or an air-CO2 mixture. The investigation was conducted through the laminar regime to analyze buoyancy ratio changes' effect on heat and mass transfers both in aiding and opposing flows. The thermal Rayleigh number was varied from 10(3) to 10(7). Streamlines, isotherms, iso-concentrations, and local and average Nusselt and Sherwood numbers are provided to demonstrate the convective flow induced. The governing equations are solved by finite volume method using SIMPLEC algorithm to handle the pressure-velocity coupling. The buoyancy ratio effect on dynamic, thermal, and mass fields is noteworthy, exhibiting both the competition between thermosolutal forces and fields' stratification. From the results, it turned out that, in general, when the buoyancy ratio is: (1) positive, thermosolutal buoyancy forces are cooperative, (2) nil, solutal buoyancy forces are weak and the flow is merely thermoconvective, (3) negative and greater than -1, buoyancy effects are competing and thermal convection dominates, (4) -1, buoyancy effects are canceled and heat and mass transfers are driven only by diffusion, and (5) less than -1, buoyancy forces compete with a dominant solutal convection.