The question above may be solved using the overall mass and component balances. If we let x be the number of liters of peroxide that should be added, the final mixture will have a quantity of x+12 L. The peroxide balance before and after mixing is,
x(1) + 12(0.08) = (x + 12)(0.16)
The value of x is 1.143. Thus, approximately 1.143 L of peroxide should be added to the original solution.