Which is the valid observation for the reaction...? #3O_2(g)rarr2O_3(g)#
#"1. The mass of the products increase."#
#"2. This is an exothermic reaction."#
#"3. The pressure of the reaction mixture should DECREASE."#
#"4. The pressure of the reaction mixture should INCREASE."#
All chemical reactions conserve mass. If you start with 10 g of reactant, from all sources, at MOST, you can get 10 g of reactant. In practice, you are not even going to get that.
The equation clearly shows that the number of gaseous molecules is reduced; nevertheless, mass is conserved:
And thus pressure is reduced due to the decrease in the number of gaseous particles.