Modern pennies are composed of zinc coated with copper. A student determines the

Question

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.481 g and then makes several scratches in the copper coating (to expose the underlying zinc). The student puts the scratched penny in hydrochloric acid, where the following reaction occurs between the zinc and the HCl (the copper remains undissolved):
Zn(s)+2HCl(aq)H2(g)+ZnCl2(aq)
The student collects the hydrogen produced over water at 25 C. The collected gas occupies a volume of 0.894 L at a total pressure of 794 mmHg .

Calculate the percent zinc in the penny. (Assume that all the Zn in the penny dissolves.)

Explanation / Answer

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

Now, pressure exerted by the gas = total pressure - aqueous tension = 794-24 = 770 mm Hg

Now, Applying the ideal gas equation, i.e. P*V = n*R*T we get

(770/760)*0.894 = n*0.0821*298

or, n = 0.037 moles

Hence from the balanced reaction, moles of Zn reacted = moles of H2 evolved = 0.037

Now, molar mass of Zn = 65.38 g/mole

Thus, mass of Zn present in the penny = moles*molar mass = 0.037*65.38 = 2.421 g

Hence % Zn = (mass of zinc/mass of penny)*100 = 97.56%

Related Questions

Navigate

• Browse (All)
• Browse M
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.