Two commonly used sulfonic acids are methanesulfonic acid (CH3SO3H) and trifluoromethanesulfonic acid (CF3SO3H). Which acid's conjugate base is the better leaving group?

trifluoromethanesulfonic acid (CF3SO3H).

Explanation:

The trifluoromethanesulfonic acid (CF3SO3H) has a halogen atom which stabilizes the leaving group by withdrawal of charge from the SO3- moiety. The methanesulfonic acid (CH3SO3H) contains an electron pushing group which tends to destabilize the charge centre. The better leaving group will be the stabilized anion which in this case is trifluoromethanesulfonic acid (CF3SO3H). This typifies the role of stabilizing factors in formation of chemical species.

Well, Here's some factors:

The concentrations of the reactants and the products.

The temperature of the system.  

And, The partial pressures of the gases involved.

Good Luck!~