Basic-laws of Chemcombi

7/27/2019 Basic-laws of Chemcombi

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In the study of matter, we observe certain

phenomena which can be reproduced at will.These observed phenomena are called

scientific facts. Often, a large number of

scientific facts can be summarized into broad

or sweeping statements called laws.


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LAW OF CONSERVATION OF MASS

-which states that when an ordinary chemicalreaction occurs, there is no detectable change in

the masses of the substances involved before and

after the reaction.

masses of the reactants = masses of the products

before the reaction after the reaction


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However, Albert Einstein, a Nobel laureate

physicist and one of the most creative intellects in

human history, recognized the quantitative

interconversion of mass and energy in reactions,as shown by his famous equation.

E = mc

2

Where: E = energy

m = mass

c = velocity of light


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The interconvertibility of mass and energy holdstrue in energetic processes like radioactivity and

atomic explosions. Strictly speaking, the

interconversion of energy and mass cannot

possibly allow a constancy of one or the other.

However, the amount of energy and mass change

are too small to be detected in the laboratory.

Therefore, we can still count on the law ofconservation of mass for practical use.


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LAW OF DEFINITE PROPORTION

-states that a pure compound always contains the

same elements combined in the same proportions

by mass.

-when two elements combine to form a given

compound, they always do so in a fixedproportion.


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To illustrate the law of definite composition, refer

to the table

Trial Mass of C

(g)

Mass of O2(g)

Mass of CO2(g)

1

2

3

2.00

15.00

5.00

5.34

40.05

13.36

7.34

55.05

18.36

These are the mass data for the amounts of C,

O2, and CO2 in the reaction.

C + O2 CO2


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Trial Mass of C

(g)

Mass of O2

(g)

Mass of CO2

(g)

1

2

3

2.00

15.00

5.00

5.34

40.05

13.36

7.34

55.05

18.36

Find the percentages of carbon and oxygen.

% of C =

x 100

% of O =

x 100


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Trial Mass of C

(g)

Mass of O2(g)

Mass of CO2(g)

1

2

3

2.00

15.00

5.00

5.34

40.05

13.36

7.34

55.05

18.36

Trial 1 Trial 2 Trial 3

C =2.0

7.34 x 100

= 27.2%

O =5.34

7.34 x 100

= 72.8%

C =15.0

55.05 x 100

= 27.2%

O =40.05

55.05 x 100

= 72.8%

C =5.0

18.36 x 100

= 27.2%

O =13.36

18.36 x 100

= 72.8%


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Trial 1 Trial 2 Trial 3

C =2.0

7.34 x 100

= 27.2%

O =

5.34

7.34 x 100= 72.8%

C =15.0

55.05 x 100

= 27.2%

O =

40.05

55.05 x 100= 72.8%

C =5.0

18.36 x 100

= 27.2%

O =

13.36

18.36 x 100= 72.8%

These show that the percentage of carbon in CO2

which is 27.2% and the percentage of oxygen inCO2 which is 72.8% are definite.

The experimental data in the example also verify

and confirm the law of conservation of mass.


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LAW OF MULTIPLE PROPORTIONS

-this states that when two elements combine toform more than one compound, the masses of

one element which combine with a fixed mass of

the other element are in a ratio of small whole

numbers such as 2:1, 1:1, 2:3, etc.


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LAW OF MULTIPLE PROPORTIONS

Sample Exercise:Two different compounds elements C and D

were found to have the following compositions:

C D

First compound 2.276 g 0.792 gSecond compound 1.422 g 0.948 g

Find the formulas of the two compound.


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Solution: Mass may be fixed at C.

C D

First compound

2.276 g/2.276g = 1 0.792 g/2.276 g = 0.348

Second compound

1.422 g/1.422 g = 1 0.948 g/1.422 g = 0.667

Therefor, the formulas of the two compounds are:Ratio C D Formula

First 1 0.348/0.348 =1 CD

compound

Second 1 0.667/0.348 = 2 CD2compound


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Practice exercises

Find the formula of the compounds having the

following composition:X Y

First compound 50 g 56 g

Second compound 35 g 82 g

Basic-laws of Chemcombi

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