Consider the following equation:
$$2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g), \Delta H = -190$$ kJ.
The number of factors which will increase the yield of SO$$_3$$ at equilibrium from the following is ______.
A. Increasing temperature
B. Increasing pressure
C. Adding more SO$$_2$$
D. Adding more O$$_2$$
E. Addition of catalyst

We have the reaction

This is an exothermic reaction with 3 moles of gaseous reactants producing 2 moles of gaseous products. We analyze each factor using Le Chatelier's principle.

Case A: Increasing temperature shifts an exothermic equilibrium backward, decreasing the yield of SO$$_3$$.

Case B: Increasing pressure favours the side with fewer moles of gas (product side with 2 moles vs. 3 moles on the reactant side), so it increases the yield.

Case C: Adding more SO$$_2$$ shifts the equilibrium forward, increasing the yield of SO$$_3$$.

Case D: Adding more O$$_2$$ also shifts the equilibrium forward, increasing the yield.

Case E: Adding a catalyst increases the rate of both forward and backward reactions equally, so it does not affect the equilibrium position or yield.

The factors that increase the yield of SO$$_3$$ are B, C, and D. Hence, the number of factors is $$3$$. So, the answer is $$3$$.