Question

The zinc within a copper-plated penny will dissolve in hydrochloric acid if the copper coating is filed down in several spots (so that the hydrochloric acid can get to the zinc). The reaction between the acid and the zinc is as follows: 2H+(aq)+Zn(s)→H2(g)+Zn2+(aq). When the zinc in a certain penny dissolves, the total volume of gas collected over water at 25 ∘C was 0.947 L at a total pressure of 740 mmHg. What mass of hydrogen gas is collected?

Answer 1

Answer: The amount of hydrogen gas collected will be 0.073 g

Explanation:

We are given:

Vapor pressure of water = 23.78 mmHg

Total vapor pressure = 740 mmHg

Vapor pressure of hydrogen gas = Total vapor pressure - Vapor pressure of water = (740 - 23.78) mmHg = 716.22 mmHg

To calculate the amount of hydrogen gas collected, we use the equation given by ideal gas which follows:

$PV=nRT$

where,

P = pressure of the gas = 716.22 mmHg  

V = Volume of the gas = 0.947 L

T = Temperature of the gas = $25^oC=[25+273]K=298K$

R = Gas constant = $62.364\text{ L.mmHg }mol^{-1}K^{-1}$

n = number of moles of hydrogen gas = ?

Putting values in above equation, we get:

$716.22mmHg\times 0.947L=n\times 62.364\text{ L.mmHg }mol^{-1}K^{-1}\times 298K\\\\n=\frac{716.22\times 0.947}{62.364\times 298}=0.0365mol$

To calculate the mass from given number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Moles of hydrogen gas = 0.0365 moles

Molar mass of hydrogen gas = 2 g/mol

Putting values in above equation, we get:

$0.0365mol=\frac{\text{Mass of hydrogen gas}}{2g/mol}\\\\\text{Mass of hydrogen gas}=(0.0365mol\times 2g/mol)=0.073g$

Hence, the amount of hydrogen gas collected will be 0.073 g