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 786 mmHg .

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

Answer 1

Solution :-

Given data

Mass of penny = 2.480 g

Volume of H2 gas collected = 0.903 L

Temperature = 25 C +273 = 298 K

Total pressure =786 mmHg

At 25 C vapor pressure of water =23.8 mmHg

Therefore the pressure of the H2 gas = total pressure – vapor pressure of water

                                                                    = 786 mmHg – 23.8 mmHg

                                                                    = 762.2 mmHg

Lets convert pressure from mmHg to atm

762.2 mmHg *1 atm / 760 mmHg =1.003 atm

Now lets calculate the moles of the H2 gas using the ideal gas law formula

PV= nRT

P =pressure , V= volume , n= moles , R= gas constant (0.08206 L atm per K mol) T= kelvin temperature

Lets plug in the values in the formula and calculate moles

1.003 atm *0.903 L = n *0.08206 L atm per K mol * 298 K

(1.003 atm *0.903 L)/ (0.08206 L atm per K mol * 298 K) = n

0.037037 mol H2 =n

Now lets calculate the moles of Zn using the moles of H2

Mole ratio of the Zn and H2 is 1 :1 therefore moles of the Zn reacted are same as moles of H2 formed

Therefore moles of Zn reacted = 0.037037 mol Zn

Now lets convert moles of Zn to its mass

Mass = moles * molar mass

Mass of Zn =0.037037 mol * 65.39 g per mol

                     = 2.4218 g Zn

Now lets calculate the percent of the Zn in the penny

% Zn = (mass of Zn / mass of penny)*100%

% Zn =(2.4218 g /2.480 g)*100 %

% Zn = 97.65 % Zn

Therefore the percent the Zn in the penny is = 97.65 %