Solved: A popular classroom demonstration consists of filing notches into a new penny

Solution 108P :

Step 1:

Given :

Mass of zinc in penny = 2.5 g

Volume of HCl = 20.0 mL

Molarity of HCl = 6.0 M

Concentration of HCl solution after all of the zinc has dissolved = ?

First, let’s write the chemical equation for the given scenario :

Zn(s) + 2HCl(aq) →  ZnCl2(aq) + H2(g)

Now, let’s calculate the moles of Zinc :

                Number of moles (n) =

Molar mass of Zinc(Zn)  = 65.38 g/mol

Therefore, n =

         = 0.0382 moles

Now, let’s calculate the moles of HCl:

        We have the molarity and volume of HCl , using this we will calculate the number of moles using the molarity formula :

                Molarity =

The formula for calculating the number of moles will be :

Number of moles = Molarity  Volume(in L)       

Volume of HCl is in mL, let’s convert to L :

                        1 L = 1000 mL

Therefore, 20 mL in L will be:

                = 20 mL

= 0.020 L      

Now, let’s calculate the number of moles :

     = 6.0 M 0.020 L

     = 0.12 mol