Question

2 moles of Zinc reacts with a cupric chloride solution containing 6.023*10^22 formula units of cucl2 calculate the moles of cu 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

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