Determining the volume occupied by one mole of Hydrogen gas

Experiment 8: TO DETERMINE/DETERMINING THE VOLUME OCCUPIED BY ONE MOLE OF HYDROGEN GAS

OBJECTIVE

TO DETERMINE THE VOLUME OCCUPIED BY ONE MOLE OF HYDROGEN GAS AT NTP (WEIGHT OF Mg = 0.03 gm)

APPARATUS REQUIRED

1. Eudiometer
2. Short stem
3. Funnel
4. Tall jar

CHEMICALS REQUIRED

1. Dil H₂SO₄
2. Water
3. Mg piece

THEORY

The equivalent mass of metal is defined as the no. of parts by mass of the metal that can displace 1.008 parts by mass of hydrogen or can combine with 8 parts by mass of the oxygen or 35.5 parts by mass of chlorine.

In this method, a known mass of the given metal is reacted completely with dil. HCl or H₂SO₄ as

Mg+H_{2}SO_{4}\rightarrow MgSO_{4}+H_{2}

Hydrogen gas displaced is collected quantitatively in an eudiometer tube by the download displacement of water. The volume of the moist hydrogen collected in the eudiometer is recorded at the laboratory temperature by equalizing the level of H₂ gas of the eudiometer in a tall jar. The moist volume of the gas is converted to dry volume at NTP by applying the ideal gas equation and law of partial pressure. Then the mass of hydrogen is calculated. The law of chemical equivalence states that the ratio of masses of elements combine or displace each other in the ratio of their equivalent masses (eq. mass).

\frac{Eq.\ mass\ of\ metal}{Eq.\ mass\ of\ Hydrogen} = \frac{Mass\ of\ metal}{Mass\ of\ Hydrogen}\\
or,\ Eq.\ mass\ of\ metal = \frac{Mass\ of\ metal}{Mass\ of\ Hydrogen} \times 1.008

PROCEDURE

A definite weight of a given piece of clean metal was taken. The metal was placed in a beaker and was covered by a short stem funnel. Water was slowly poured down into a beaker so that the funnel’s stem was completely under water. Now, an eudiometer tube was taken which was half filled with dilute sulphuric acid and the other half was filled with water up to the brim. It was closed with the thumb. Now, the tube was inverted over the funnel stem under water. The hand was removed carefully.

The eudiometer tube was clamped in the vertical position as shown in the figure. The reaction between acid and metal took place. When the reaction was completed, the mouth of the eudiometer tube was closed with the thumb under water. It was removed and immersed in an inverted position underwater in a tall jar containing water. Then the level of the water bath inside and outside of the tube was equalized and the volume of the moist hydrogen gas was read. This is called the levelling method. The temperature of water in the tall jar was noted and pressure was also noted.

OBSERVATION

Weight of metal taken = 0.03 gm
Volume of hydrogen gas in eudiometer tube = 32 ml Temperature of water = 298K
Atmospheric pressure = 760 mm Hg
Aqueous tension at 25 °C = 23.8 mm Hg
Pressure of dry hydrogen = 760 – 23.8 = 736.2 mm Hg

CALCULATIONS

From the combined gas equation,

\frac{P_{1}V_{1}}{T_{1}} = \frac{P_{2}V_{2}}{T_{2}}\\
V_{2} = \frac{P_{1}V_{1}T_{2}}{P_{2}T_{1}}\\
= \frac{736.2 \times 32 \times 273}{760 \times 298}\\
= 28.39\ mL

By chemical reaction,

\underset{1\ Mole}{Mg}+H_{2}SO_{4} \rightarrow MgSO_{4}+\underset{1\ Mole}{H_{2}}

1 mole of Mg displaces 1 mole of hydrogen gas.
(0.03/24) mole of Mg displaces (0.03/24) mole of hydrogen gas

So,

(0.03/24) mole of hydrogen gas = V₂ ml of hydrogen gas at NTP
1 mole of hydrogen gas = (V₂ x 24)/0.03 ml of hydrogen gas at NTP
= (28.39 x 24)/0.03 ml of hydrogen gas at NTP
= 22712 ml of hydrogen gas at NTP
= 22.712 litre of hydrogen gas at NTP

RESULT

The volume occupied by one mole of hydrogen gas at NTP was found to be 22.712 litres.

CONCLUSION

The volume occupied by one mole of hydrogen gas at NTP was determined by the hydrogen displacement method and this value agrees with the theoretical molar volume (22.4 lit).

PRECAUTION

1. Air bubbles should be passed while inverting or removing the eudiometer tube.
2. Reaction should be complete.
3. Dilute nitric acid should not be taken.
4. The Stem of the funnel should be short.