To verify the law of conservation of mass in a chemical reaction - Lab Work

To verify the law of conservation of mass in a chemical reaction - Science Practicals

Aim
To verify the law of conservation of mass in a chemical reaction.

Theory
Law of conservation of mass states that the mass remains conserved during a chemical reaction . In this experiment we shall verify the law of conservation of mass u sing a precipitation reaction. This reaction is considered as the simplest method to verify this law.

Materials Required
Barium chloride (BaCl2.2H2O), sodium sulphate (Na2SO4.10H2O), distilled water, two beakers (150 mL), one beaker (250 mL), physical balance, spring balance (0 – 500 g) and a polythene bag, two watch glasses of known masses and a glass stirrer.

Procedure
Step 1: 100 mL distilled water is poured in two beakers (150 mL).
Step 2: The physical balance and a watch glass of known mass are used, weigh 7.2 g of BaCl2 .2H2O and dissolved in a beaker (150 mL) containing 100 mL distilled water.
Step 3: Similarly, 16.1 g of Na2SO4.10H2O is weighed in another watch glass of known mass and dissolved in another beaker (150 mL) containing 100 mL distilled water.
Step 4: The third beaker (250 mL) is taken and weighed using a spring balance and polythene bag.
Step 5: Both solutions of 150 mL beakers is mixed in the third beaker (250 mL). The contents are mixed using a glass stirrer.
Step 6: White precipitate of BaSO4 appears on mixing due to precipitation reaction.
Step 7: The beaker containing the reaction mixture is weighed again to determine the mass of the precipitation reaction products.
Step 8: The masses of before and after the chemical reaction are compared.

Observations
(i) Mass of 100 mL distilled water = 100.0 g (The density of distilled water is 1 g/mL.)
(ii) Mass of BaCl2 .2H2O = 7.2 g
(iii) Mass of BaCl2 solution = 107.2 g
(iv) Mass of Na2 SO4.10H2O = 16.1 g
(v) Mass of Na2SO4 solution = 116.1 g
(vi) Total Mass of reactants = 223.3 g (solutions of BaCl2  and Na2SO4)
(vii) Mass of empty 250 mL beaker, m1 = _____ g
(viii) Initial mass of reaction mixture and empty beaker (before the precipitation), m2 = (m1 + 223.3 g) = _____ g
(ix) Final mass of reaction mixture in the beaker after the precipitation, m3 = _____ g.

Results and Discussions
Compare the initial mass (m2) of the reaction mixture (before the precipitation) with the final mass (m3) of the reaction mixture (after the precipitation). If the two masses are same within the reasonable limits, then the law of conservation of mass stands verified.
The verification of the law rests on accurate mass measurements in the laboratory. The chemical reaction involved is:
BaCl2 (aq) + Na2SO4 (aq) ⎯→ BaSO4 (s) + 2NaCl (aq)
                                           White precipitate
and more precisely
Ba2 + (aq) + SO42- (aq) ⎯→ BaSO4 (s)

Precautions
1. The spring balance should be held vertical while taking measurements.
2. Before making use of the spring balance it must be ensured that its pointer is at zero mark. If not then ask your teacher to help.
3. The readings of the spring balance should be noted only when its pointer comes to rest.
4. Mixing of barium chloride and sodium sulphate solutions be done slowly with constant stirring.

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