Acommon 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 144.2 ml of gas at a pressure of 742 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.)

Data:

Volume = 144.2 ml = 0.1442 l
total pressure = 742 mmhg
temperature = 25°C.
mass of hydrogen gas (in mg) = ?
vapor pressure of water is 23.78 mmhg at 25°C

Solution:

1) Partial pressure of hydrogen = total pressure - vapor pressure of water

Partial pressure of hydrogen = 742 mmHg - 23.78 mmHg = 718.22 mmHg

2) Number of moles of hydrogen

pV = nRT => n = (pV) / (RT)

R = 62.36367 mmHg * l / mol * K

n = (718.22 mmHg * 0.1442 l ) / (62.36367 mmHg*l /K*mol * 298.15K) = 0.00557 mol

3) mass of hydrogen (H2)

mass = number of moles * molar mass

molar mass of H2 = 2 * 1 g/mol = 2 g/mol

=> mass = 2 g/mol * 0.00557 mol = 0.01114 g. = 11.14 mg

11.14 mg

Density and pressure