[eureka_beyond]

A) i do not understand the term "theoretical mass"
B) Question: What is theoretical mass of copper obtained when 17.4g of copper (II) oxide is heated with excess carbon ?
 
Thanks! 

[Mitch]

In this case, it means the maximum mass of copper obtained from the reaction.

[kinerd]

A) Theoretical mass is the same term for theoretical yield. The theoretical mass is how much product will be formed in ideal conditions (basically an estimate). Usually the reaction does not occur under ideal conditions, and the reaction will not fully produce the expected amount. What is actually obtained is called the actual yield.
B) Write a balanced equation describing the reaction: the reactants are copper(II) oxide and carbon, find the products. Then convert the 17.4g of CuO into moles of CuO. Use the mole ratio of CuO to Cu to convert the moles of CuO into moles of Cu. Then, convert the moles of Cu into grams of Cu using the molar mass. That is your theoretical mass of copper.