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 3.0 g of zinc and is soaked in 25.0 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+. Express your answer using two significant figures.

Explanation / Answer

Solution :-

Balanced reaction equation

Zn + 2HCl -----> ZnCl2 + H2

Lets calculate the moles of Zn

moles of Zn = mass/ molar mass

                    = 3.0 g / 65.39 gper mol

                    = 0.0459 mol

lets find the moles of HCl needed to react with Zn

0.0459 mol Zn * 2 mol HCl / 1 mol Zn = 0.0918 mol HCl

now lets find total moles of HCl in 25 ml of 6.0 M HCl

moles of HCl = molarity * volume in Liter

                       = 0.15mol

now lets find the moles of HCl remain after reaction

moles of HCl remain = 0.15 mol - 0.0918 mol = 0.0582 mol

now lets find the molarity of HCl remain after reaction

molarity of HCl = 0.0582 mol / 0.025 L

                         = 2.328 M

So the concnetration of HCl remain after reaction = 2.328 M

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