Question

2 moles of zinc reacts with a cupric chloride solution containing 6.023 x 10*22 formula units of CuCl2. calculate the moles of copper obtained. Zn + CuCl2 ->ZnCl2 + cu

Answer 1

First write down the equation that takes place:

          Zn +  CuCl2 ---------->  ZnCl2 + Cu

There 1 mole of Zn will react with 1 mole of CuCl2 to give 1 mole of Cu.

From the question we have 2 moles of zinc, and 
CuCl2 has 6.023x10^22 formula units(calculate its moles)

If       6.023x10^23 (avagadro's constant) = 1 mole
then, 6.023x10^22                       =  1 mole x 6.023 x10^22/6.023x10^23
                                                     =  1 mole x 10^22/10^23
                                                     = 1 mole x 10^-1
                                                      =  0.1 moles
Find the limiting factor as not all the Zinc will react with the cupric chloride:
Since 1 mole of zinc requires 1 mole of CuCl2 to give 1 mole of copper, while in this case there is only 0.1 moles of CuCl2, then CuCl2 is the limiting factor and is what we use for the calculation as follows:

If 1 mole of CuCl2 gives 1 mole of Cu.
Then how much  will 0.1 mole of CuCl2 give = 1mole x 0.1 mole/1mole
                                                                       = 0.1 moles

Therefore moles of Cu obtained is 0.1 moles

Answer 2

given in the question moles of zinc =  2 and moles of copper chloride = 6.023 x 10^22 /  6.023 x 10 ^23 =  0.1 mole 
now according to the question i mole of Zn requires 1 mole of copper chloride therefore in this reaction 0.1 mole of copper chloride  will require only 0.1 mole of Zn hence copper chloride is the limiting reagent 
therefore according to equation 0.1 mole of copper chloride produces  0.1 mole of copper as 1mole of copper chloride gives 1 mole of copper.