if 4.2 moles of copper metal react with 6.3 moles of silver nitrate, how many moles of silver metal can be formed, and how many moles of the excess reactant will be left over when the reaction is complete?   unbalanced equation: cu + agno3  → cu(no3)2  + ag  be sure to show all of your work.

The balanced reaction for the above reaction is;
Cu + 2AgNO₃ ---> Cu(NO₃)₂  +  2Ag
we need to first find the limiting reactant 
stoichiometry of Copper to AgNO₃ is 1:2
the number go Copper moles are 4.2 mol
the number of AgNO₃ moles are 6.3 mol
if AgNO₃ is the limiting reactant 
if 2 mol of AgNO₃ reacts with 1 mol of Cu 
then 6.3 mol of AgNO₃ reacts with - 1/2 x 6.3 mol = 3.15 mol of Cu
and 4.2 mol of Cu is present therefore AgNO₃ is the limiting reactant and Cu is in excess. 
stoichiometry of AgNO₃ to Ag is 2:2
therefore number of AgNO₃ moles reacted = number of Ag moles formed
number of Ag moles formed = 6.3 mol

excess reactant is copper 
only 3.15 mol were used up but 4.2 mol were present initially
the amount of excess reactant moles used = 4.2 - 3.15 = 1.05 mol of copper is in excess