MOLAR VOLUME OF HYDROGEN

Background Information

The term, molar volume, refers to the volume of one mole of gas. Since volume varies with temperature and pressure, it would be meaningless to compare volumes measured under different conditions. It is therefore customary to make the measurements under convenient laboratory conditions and then convert the measurements to the volume at standard temperature and pressure (STP, 0°C and 1 atm).

For an ideal gas, the size of a gas sample is insignificant compared to the size of the container holding the gas, and there is no interparticle interaction (no attraction nor repulsion between particles). The molar volume of the gas at a particular temperature and pressure is independent of the type of gas. 

In this experiment, hydrogen gas generated by the reaction of magnesium with hydrochloric acid is collected over water. 

Mg(s) + 2 HCl(aq)   →  MgCl₂(aq) + H₂(g) 

The volume of the gas is measured and the number of moles of gas is calculated from the mass of the magnesium strip used. By dividing the volume by the number of moles we obtain the molar volume at the temperature and pressure at which the experiment is performed. In order to find the molar volume at STP, we apply the Ideal Gas Law:

P V = n R T 

where P = the pressure of the gas 

V = the volume of the gas 

n = the number of moles of gas 

R = gas constant 

T = the temperature in K

Collecting a Gas Over Water

Collecting a gas over water is a common technique to isolate and study gases produced during chemical reactions. It allows for the collection of gases that are either insoluble or sparingly soluble in water. The technique relies on the principle that the gas displaces water from a container, allowing it to be collected and examined.

The procedure typically involves using a gas collection apparatus, which consists of a reaction vessel connected to an inverted container filled with water. The reaction vessel is equipped with a delivery tube that extends below the water surface in the container. As the gas is generated, it displaces the water in the container, causing the gas to rise and collect in the inverted container. The water level in the container acts as a barrier, preventing the gas from escaping while allowing it to displace the water.

It is important to note that certain precautions must be taken when collecting gases over water. For example, it is essential to ensure that the reaction vessel is airtight and that the gas being collected is not excessively soluble or reactive with water. Additionally, temperature and atmospheric pressure should be taken into account to accurately measure the volume of the collected gas.

Dalton's Law of Partial Pressures

This law states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of each individual gas. According to this law, each gas in the mixture behaves independently and exerts its own pressure as if it were the only gas present. The partial pressure of a gas is directly proportional to its mole fraction in the mixture, which is the ratio of the number of moles of that gas to the total number of moles of all gases in the mixture. Dalton's law is particularly useful in understanding and predicting the behavior of gas mixtures in various applications, such as in industrial processes, gas analysis, and the study of atmospheric gases.