This method is used to determine the equivalent mass of those metals such as magnesium, zinc and aluminium which react with dilute acids and readily displace hydrogen.
Mg + 2HCl -> MgCl2 + H2
Zn + H2SO4 - > ZnSO4 + H2
2Al + 6HCl - > 2AlCl3 + 3H2
From the mass of the metal and the volume of hydrogen displaced, the equivalent mass of the metal can be calculated.