The law of mass conservation is another fundamental law in chemistry. This law s

Question

The law of mass conservation is another fundamental law in chemistry. This law states that matter can neither be created nor destroyed, which means that in a chemical traction where all of the reactant is converted into product, the mass of reactant that reacts must be exactly equal to the mass of product produced. Next work you are going to test the law of mass conservation. To do this you will need to design your own experiment. Consider the 2 precipitation reactions you have already studied this semester 3CaCl2 + 2Na3PO4 right arrow Ca3(PO4)2 + 6NaCl Na2CO3+ CaCl2 right arrow CaCO3 + 2NaCl Think about an experimental procedure that you can use next week Remember that you are trying to collect data that can be used to illustrate the law of mass conservation. Contemplate how you might visually represent this data in the form of a graph. What are the variables that you might need to plot. How would your choice of graph illustrate the law of mass conservation.

Explanation / Answer

Law of conservation of mass

(a) we have,

3CaCl2 + 2Na3PO4 ---> Ca3(PO4)2 + 6NaCl

mass of reactant = (3 x Ca + 2 x Cl) + (6 x Na + 2 x P + 8 x O)

                            = (3 x 40.078 + 2 x 35.453) + (6 x 22.9898 + 2 x 30.9738 + 8 x 15.9994)

                            = 519.0216 g

mass of products = (3 x Ca + 2 x P + 8 x O) + (6 x Na + 2 x Cl)

                            = 519.0216 g

So mass of reactants = mas of products

Next we have,

Na2CO3 + CaCl2 ---> 2NaCl + CaCO3

count the number of atoms on both sides,

           Reactant      Product

Na           2                  2

C             1                  1

O             3                  3

Ca           1                  1

Cl            2                  2

So we see we have same number of atoms of each element on both reactant and product side.

this proves the law of conservation of mass

(b) Experimentally we can do a closed vessel reaction and observe the masses of the starting material and that of the product.

Take for example lead nitrate reaction with potassium iodide

Pb(NO3)2(white) + 2KI(white) ---> PbI2 (yellow ppt.) + KNO3 (white)

In a clean dry weighed flask add weighed amounts of Pb(NO3)2 and KI in weighed amount of water.

The two white solids react to give a yellow precipitate of PbI2.

Weight the total mass of flask+content before and after the reaction. You should be able to see no change in the total mass. This proves the law of conservation of mass.

Alternately a simple experiment is to take ice in a vessel and melt it to water. Measure the starting weight and end weight of flask+sample. No change in weight should be observed, which proves law of conservation of mass.

(c) To prove law of conservation of mass by graph plot, mass of reactants on x axis and mass of product on the y-axis. At the start of the reaction, no product is present and only starting material exists. Take that as the initial weight of reaction. As the reaction progresses, the mass of starting material decreases and the mass of product increases. At any given point the total mass of reactant and product equals the intial mass. At the end of reaction, we only have product in the reaction with no reactant left. Mass of product equals initial mass of reactant taken from graph.

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