Lesson 8, Topic 2
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# Law of Conservation of Mass (Matter)

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Lavoiser, a French chemist observed in 1774 that the mass of reacting substances was the same as the mass of the product.

The law of conservation of mass (matter) states that in an ordinary chemical reaction, matter can neither be created nor destroyed. This means that in a chemical reaction the total mass of all the reacting substances (ie the reactants) is equal to the total mass of the products.

Example I

Sodium chloride reacts with silver trioxonitrate (V) to produce silver chloride and sodium trioxonitrate (V). Na = 23, Cl = 35.5, Ag = 108, N = 14, O = 16.

Show that these reactions verify the law of conservation of mass.

Equation of the Reaction

Example II

20.10g of sodium chloride were made to react with 15.80g of Lead(II) trioxonitrate(V) to produce 12.40g of Lead(II) chloride precipitate and 23.50g of sodium trioxonitrate(V). Show that these reactions verify the Law of conservation of matter.

Solution

Equation of Reaction

From examples I and II, since the mass of the products are equal to the mass of the reacting substances, the law of conservation of mass or matter is verified.

### Experiment to Verify the Law of Conservation of Mass:

Procedure:

1. Put some silver trioxonitrate(V) (AgNO3) solution into a small conical flask. Lower into the flask a small test tube containing sodium chloride (NaCl), by means of a thread. Cover the flask with a stopper.

2. Weigh the entire set-up on a chemical balance, and record the mass.

3. Tilt the flask allowing the sodium chloride in the test tube to mix with silver trioxonitrate (V) solution in the flask. White precipitate of silver chloride AgCl will be obtained.

$$\scriptsize AgNO_3 \: + \: NaCl \: \rightarrow \: \underset {white \: precipitate}{ AgCl} \: + \: NaNO_3$$

Leave the apparatus for some time and reweigh the whole apparatus.

4. It will be observed that the weight of the apparatus before the reaction equals the weight of the apparatus after the reaction.

Other examples

$$\scriptsize BaCl_2 \: + \: H_2SO_4 \: \rightarrow \: \underset {precipitate}{ BaSO_4} \: + \: 2HCL$$

$$\scriptsize Pb(NO_3)_2 \: + \: 2NaCl \: \rightarrow \: \underset {precipitate}{ PbCl_2 }\: + \: 2NaCl$$

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