Determining the weight of a metal by Hydrogen Displacement Method

Experiment 7: TO DETERMINE/DETERMINING THE WEIGHT OF A METAL BY HYDROGEN DISPLACEMENT METHOD

OBJECTIVE

TO DETERMINE THE WEIGHT OF A GIVEN PIECE OF METAL BY THE HYDROGEN DISPLACEMENT METHOD

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 combines or displaces each other in the ratio of their equivalent masses (eq. masses).

\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 stem of the funnel 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 stem of the funnel 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 under the water 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

Volume of hydrogen gas in eudiometer tube = 32 ml Temperature of water = 298K
Atmospheric pressure = 760 mm Hg
Aqueous tension at 25oC = 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

we know,

\begin{align*} 
wt.\ of\ metal &= \frac{eq.\ wt \times V_{2}\times 0.000089}{1.008}\\
&= \frac{12 \times 28.39\times 0.000089}{1.008}\\
&= 0.03\ gm
\end{align*}

RESULT

The weight of the given metal was found to be 0.03 gm.

CONCLUSION

The weight of a given piece of metal was determined by the hydrogen displacement method.

PRECAUTION

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