Answer:
The answer to your question is: y = 66.7g
Step-by-step explanation:
To solve this problem we must remember the Lavoisier law of conservation of mass, that stated that the mass can not be created or destroyed but it changes forms.
So, the mass of the reactants is equal to the mass of the products.
Process
x + y ⇒ xy
12.2g ? 78.9 g
12.2 g + y = 78.9g
y = 78.9 - 12.2
y = 66.7g