Molar volume is the name given to the volume that one mole of any gas occupies at standard temperature and pressure. In chemistry, many of the materials worked with are gases. It is often easier to measure the volume of a sample of gas, rather than determine its mass. The main purpose of this lab is to determine the molar volume of hydrogen gas experimentally in order to compare it to the theoretical molar volume of ideal gases. In this experiment, a known mass of magnesium is reacted with an excess amount of hydrochloric acid to produce hydrogen gas. The amount of hydrogen gas produced is collected by the displacement of water. This volume is used to calculate the molar volume of hydrogen gas. This data can then be used to compare the experimental molar volume of hydrogen gas and that of ideal gases at STP (standard temperature and pressure), by first using the Combined Gas Law (P1V1/T1=P2V2/T2) to find the experimental molar volume at STP, followed by the Ideal Gas Law (PV=nRT) to find the molar volume of any gas at STP. The partial pressure of the hydrogen gas must be found before the combined gas law can be used. It is found using Dalton's Law of Partial Pressure (P=PH2+PH2O). Data and Results
Mass of Mg Used
Volume of Gas Collected
Water Vapor Pressure 0.0145 g
Combined Gas Law: P1V1/T1=P2V2/T2
Used to find the molar volume of hydrogen gas at STP.
Ideal Gas Law:
PV = nRT
(101.3 kPa)(V) = (1 mol)(8.314 L.kPa/mol.k)(273 K)
V = 22.4 L
Used to find the molar volume of any gas at STP
% Error: 100 x (theoretical value - experimental value)/ (theoretical value) Used to find the percent error.
Dalton's Law of Partial Pressure: P=PH2+PH2O
Used to find the partial pressure of the hydrogen gas.
Molar volume of H2
Pressure of H2
Molar volume of H2 at STP
% error 0.0017 mol
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