2Cu + 4HNO3 ---->Cu(NO3)2 + 2H2O + NO2
moles of Cu = 7.87X10^-3 (this have done correct)
you are right that Cu is the limiting reactant
now you want to know molarity
so molarity = moles /volume
you have moles of Cu(NO3)2 and volume is the same as the volume of the soln (5mL) .Do not forget to change volume in L Now you can plugin the vlaues to get the molarity.
molarity X volume will give you moles of Cu2+ and then that can be converted into grams of Cu .