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

Question

A popular classroom demonstration consists of filing notches into a new penny and soaking the penny in hydrochloric acid overnight. Since new pennies are made of zinc coated with copper, and since hydrochloric acid dissolves zinc and not copper, the inside of the penny is dissolved by the acid, while the outer copper shell remains. Suppose the penny contains 1.0 g of zinc and is soaked in 20.0 mL of 6.0 M HCl.

Calculate the concentration of the HCl solution after all of the zinc has dissolved.
Hint: The Zn from the penny is oxidized to Zn2+.

Explanation / Answer

Zn(s) + 2HCl(aq) ------------> ZnCl2(aq) + H2(g)

moles of Zn = 1.0 g / 65.38 = 0.0153

moles HCl = molarity x Litres

                  = 6.00 M x 0.0200 L

                   = 0.12

According to the balanced equation
1 mole Zn will react ----------------->   2 moles of HCl

0.0153 moles of Zn -----------------> ??   moles HCl

moles of HCl = 0.0153 x 2 = 0.0306

Thus the 1.0 g of Zn will use up 0.0306 moles of HCl

The moles of HCl remaining in solution = total moles HCl - moles HCl used up
                                                                   = 0.12 moles - 0.0306 moles
                                                                    = 0.0894 moles HCl left over

Molarity = moles HCl / Litres solution
= 0.0894 mole / 0.0200 L
= 4.47 M

Molarity = 4.5 M

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