Question

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.480 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 0.903 L at a total pressure of 785 mmHg .

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

Answer 1

Answer:

The percent of zinc in the penny is 96.84%.

Explanation:

Volume of the hydrogen gas = 0.903 L

Pressure of hydrogen  gas = P

P = total pressure  - Water's vapor pressure at 25°C

P = 785 mmHg - 23.8 mmHg = 761.2 mmHg

1 atm = 760 mmHg

761.2 mmHg = 1.0016 atm

Temperature of the gas = T = 25°C = 298.15 K

Moles of hydrogen gas collected = n

$PV=nRT$ (ideal gas equation )

$n=\frac{PV}{RT}=\frac{1.0016 atm\times 0.903 L}{0.0821 atm L/mol K\times 298.15 K}=0.03695 mol$

$Zn(s)+2HCl(aq)\rightarrow H_2(g)+ZnCl_2(aq)$

According to reaction , 1 mol of hydrogen gas is given by 1 mol of zinc.

Then 0.03695 moles of hydrogen gas will be given by:

$\frac{1}{1}\times 0.03695 mol=0.03695 mol$ of zinc

Mass of 0.03695 moles of zinc = 0.03695 mol × 65 g/mol= 2.4016 g

Percentage of zinc in 2.480 grams of penny:

$\frac{2.4016 g}{2.480 g}\times 100=96.84\%$