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 152.2 mL of gas at a pressure of 741 mmHg and a temperature of 25∘C. What mass of hydrogen gas (in milligrams) does the student collect? (The vapor pressure of water is 23.78 mmHg at 25∘C.)

Answer 1

Answer :

Given that :

Volume collected( V) = 152.2 ml

Temperature (T) = 25°C = 298 K

Total pressure = 741 mm Hg

Pressure due to H_{​​​​​​2} gas (P) = total pressure - vapor pressure of water = 741 - 23.78 = 717.22 mmHg

Pressure in atm = 717.22 / 760 = 0.9437 atm

R (gas constant ) = 0.0821 L atm mol^{​​​​-1} K^{​​​​​​-1}

n (moles ) = PV / RT = 0.9437 * 0.152 / (0.0821 * 298 ) = 5.86 mmoles

Mass of H_{​​​​​​2} gas (mg) = n (mmoles ) * M(Molar mass of H_{​​​​​​2} gas = 5.86 * 2 = 11.72 mg