I am studying for a midterm, I got the answer right to this question, but could someone explain why? thank you :)

Mass of product formed =  7.49 g

Explanation:

According to the law of conservation mass, mass can neither be created nor destroyed in a chemical equation.

This law was given by French chemist  Antoine Lavoisier in 1789. According to this law mass of reactant and mass of product must be equal, because masses are not created or destroyed in a chemical reaction.

For example:

In given photosynthesis reaction:

6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂

there are six carbon atoms, eighteen oxygen atoms and twelve hydrogen atoms on the both side of equation so this reaction followed the law of conservation of mass.

In given question it is stated that the amount of reactant A is 7.54 g and the amount of reactant B is 6.55 g so it means total amount of reactant is

A + B    =   14.09 g

It is also given that after the completion of reaction the amount of reactant A left is 3.82 g and amount of B left is 2.78 g.

Thus total amount left is 2.78g + 3.82g = 6.6 g

Mass of product formed = total amount of reactant - amount left

Mass of product formed =  14.09 g - 6.6 g

Mass of product formed =  7.49 g