A common way to make hydrogen gas in the laboratory is to place a metal such as

Question

A common way to make hydrogen gas in the laboratory is to place a metal such as zinc in hydrochloric acid (see the figure). The hydrochloric acid reacts with the metal to produce hydrogen gas, which is then collected over water. Suppose a student carries out this reaction and collects a total of 156.0 mL of gas at a pressure of 749 mmHg and a temperature of 25° C. What mass of hydrogen gas (in mg) does the student collect? (The vapor pressure of water is 23.78 mmHg at 25 C.) Collecting a Gas over Water Hydrogen plus water vapor -Zn HCl H20 Express the mass to three significant figures and include the appropriate units.

Explanation / Answer

First calculate no. of mole of H2 gas collected

Use ideal gas equation for calculation of mole of gas

Ideal gas equation

PV = nRT             where, P = atm pressure= 749 - 23.78 = 725.22 mmHg = 0.9542 atm,

V = volume in Liter = 156 ml = 0.156 L

n = number of mole = ?

R = 0.08205L atm mol-1 K-1 =Proportionality constant = gas constant,

T = Temperature in K = 250C = 273.15+ 25 = 298.15 K

We can write ideal gas equation

n = PV/RT

Substitute the value

n = (0.9542X 0.156)/(0.08205 X 298.15) = 0.00608 mole

mole H2 gas collected = 0.00608 mole

molar mass of H2 gas = 2.01 gm/mole then 0.00608 mole of H2 gas = 0.00608 X 2.01 = 0.01223 gm

0.01223 gm = 12.23 mg

12.23 mg H2 gas collected

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