Nickel was reacted with an aqueous solutionn containing 400grams of copper(II)perchlorate. The reaction was stopped after 1 hour and 89.0 g of copper metal was obtained. Calculated the percent yeild?

You must first determine the theoretical yield. That is a regular stoichiometry problem.

Write and balance the equation, convert 400 g copper(II) perchlorate to moles, then convert moles of the perchlorate to moles of the product(Cu). Finally, convert moles of copper to grams. grams = moles x molar mass. This is the theoretical yield.

%yield = (actual yield/theoretical yield)*100 = ??
The actual yield is given in the problem as 89.0 grams Cu.