A12.2-g sample of x reacts with a sample of y to form 78.9g of xy. what mass of y reacted?

The answer to your question is:  y = 66.7g

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

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the answer is 303.15 or 303