Solution A has a mass of 70 g. Solution B has a mass of 35 g. When they are mixed, a chemical reaction occurs in which a gas is produced. If the mass of the final mixture is 90 g, what mass of gas was produced?

 Mass of gas produced : 15 g

In general, the reaction takes place in the open air. If the reaction results in the form of gas as in combustion, the mass of the reaction results will be smaller than the original mass.  

Conservation of mass applies to a closed system, where the masses before and after the reaction are the same

So In a closed/isolated system, the total mass of the substance before the reaction will be equal to the total mass of the reaction product.

Mass of solution A  = 70 g

Mass of solution B = 35 g

Mass of mixture : 90 g

Mass of gas : X

Mass of gas X :