TO DETERMINE THE EQUIVALENT WEIGHT OF THE GIVEN METAL BY HYDROGEN DISPLACEMENT METHOD
APPARATUS REQUIRED:
i. Beaker
ii. Eudiometer Tube
iii. Short Stem Funnel
iv. Thread
v. Stand with a clam
vi. Tall Jar
CHEMICALS REQUIRED:
i. Mg metal
ii. Acid (either dil. HCl or dil. H2SO4)
THEORY
Equivalent weight is defined as the number of parts by a weight of which combines or displace 1.008 parts by weight of hydrogen, 8 parts by weight of oxygen and 35.5 parts by weight of chlorine. The law of equivalents states that one equivalent of a substance reacts with one equivalent of another.
One gram equivalent of a substance = ((Mass of substance in gram)/(Equivalent wt. of the substance))
Among different methods of determination of equivalent weight, hydrogen displacement method is one of the important method. This method is applied for those metals which can displace hydrogen from dilute acid i.e. more electropositive metal which are above hydrogen in the electrochemical series. For example, Mg, Zn, Fe, etc.
Mg + dil. HCl → MgCl2 (aq) + H2(g) ↑
Hydrogen displaced from the dilute acid by a metal (Mg) whose equivalent weight is to be determined, is collected in a eudiometer tube over water. The moist volume of the H¬2 gas is converted in to dry volume at NTP and the mass of given volume of H2 is calculated. Finally, mass of given metal is determined using the following relation.
(Mass of metal)/(Mass of H_2 )=(Equivalent weight of metal)/(Equivalent weight of hydrogen)
The displacement reaction between metal and acid is redox reaction in which acid is oxidizing agent but it’s oxidizing action should be mild, otherwise hydrogen as reducing agent would react further with acid.
H2SO4 and HNO3 cannot be used of any concentration in this method because they are powerful oxidizing agent. When concentrated sulphuric and nitric acid are reacted to the metals, gases like SO2 and different oxides of nitrogen are produced respectively.
Zn + 2H2SO4 (conc.) -> ZnSO4 + 2H2O + SO2 (g)↑
Zn + 2HNO3 (conc.) -> Zn(NO3)2+ 2H2O + NO2 (g)↑
This method is not used for less electropositive metals i.e. those metals which lie below hydrogen in the electrochemical series such as Cu, Hg, Ag, etc.
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PROCEDURE:
The given piece of Mg metal was taken in big beaker containing water tied by thread. It was then, covered with short stem funnel. The stem of funnel should remain immersed in water layer. Eudiometer tube was taken and half of it was filled with provided acid and half with water up to the rim. The mouth of the tube was closed with the thumb and by inverting it was placed on the stem of the funnel as shown in the figure. As the reaction occurred, the hydrogen gas so produced was collected in eudiometer tube by downward displacement of water. The whole Mg piece was completely dissolved, then volume of gas collected was noted by lowering it in a tall jar full of water, till the level of water inside and outside become equal.
OBSERVATIONS:
In Lab
Volume of moist H2 displaced (V) = 29.5 cc
Temperature (T) of lab= (273 +t) =(273+20)=293 K
Aqueous Tension (f) at t℃ = 17.41 mm of Hg
Laboratory pressure (P) = 640 mm of Hg
Pressure of dry gas (Pd) = P – f = 622.59 mm of Hg
Volume of hydrogen gas displaced (V2) =29.5 cc
For NTP Conditions
Volume of hydrogen gas displaced (V2) =?
Temperature (T2) = 273 K
Pressure (P2) = 760 mm of Hg
By using the combined gas equation,
(P_1 V_1)/T_1 =(P_2 V_2)/T_2
or,(622.5929.5)/293=(760V_2)/273
or,17112.73=760V_2
∴V_2=22.51 cc
Mass of 1cc of dry H2 gas at NTP = 0.000089 g
Mass of V2 cc of dry H2 gas at NTP = 0.000089V2 = 0.00008922.51 = 0.002 g
So, now
(Mass of hydrogen displaced)/(Equivalent wt. of hydrogen)=(Mass of metal)/(Equivalent wt. of metal)
or,(0.002 )/1.008=0.03/E
or,E=(1.0080.03)/0.002
∴ E=15.12
RESULT
Hence the equivalent wt. of more electropositive metal Mg is found to be 15.12 from the experiment. This value is close to the actual value of equivalent wt. of Mg which is equal to 12.
CONCLUSION
Thus, from this experiment we can conclude that, the equivalent wt. weight of metal more electropositive than hydrogen can be found by hydrogen displacement method.
PRECAUTIONS
i. This method can be carried out for those metals more electropositive than hydrogen.
ii. Dilute acids should be used.
iii. The volume of hydrogen gas should be measured carefully.
iv. Completion of reaction should be checked properly.
v. Apparatus should be handled carefully.
References: