What is the theoretical yield of C6H5Br if 42.1g of C6H6 react with 73.0g of Br2?

how do you calculate the theoretical yield?
and
why is the molar mass 0f Br2 73.0g instead of 160g(using the rounded periodic table)?

Why does the equilibrium position of the spring change when a mass is added to the spring? will the mass oscillate around the new equilibrium position of the spring or the previous position without a mass attached to the spring? if the equilibrium position of the spring changes by 20 cm (assuming no initial mass) when a mass is added to the spring with constant 4.9 kg/s^2, what is the mass of the object attached to the spring?