### Determination of equivalent weight of metal by hydrogen displacement method

This method is used to determine the equivalent mass of those metals (Na, Zn,Al, and Mg) which readily displace hydrogen from an acid or from a base or water.

Mg + 2 HCl ? MgCl2 + H2

Procedure:
? In this method a known mass of metal m (g) is added to acid or base or water.
? The volume of amount of hydrogen liberated (V1) is measured at room temperature and pressure.
? The pressure of moist hydrogen gas displaced (P) is measured.
? From the aqueous tension (p), the pressure of dry hydrogen gas displaced (P1) is measured.
? Using the values of standard pressure (P0) and standard temperature (T0) and room temperature (T1) the volume of hydrogen gas displaced is converted into volume of hydrogen gas displaced at STP by the gas equation

P1V1 P0V0
------ = -------
T1 T0

Calculation:
The mass of the metal displaced by V0 m3 of the dry hydrogen gas at STP = m.g
The mass of the metal displaced by 1.12 x 10-2 m-3
m
of the dry hydrogen gas at STP = -------------- = × 1.12 × 10–2m3
V0 m3

The mass of 1.12 x 10-2 m3 of at STP = 1.008g

(hence by definition it represents the equivalent mass of the metal)

Mass of the metal x 1.12 x 10-2 m
Equivalent mass of the metal = ----------------------------------------
V0m3