Law of constant proportion

The law was proposed by a French chemist joseph L. Proust. In 1797 and hence it also referred to proust’s law.

Law State:

In a chemical substance elements are always in fixed proportion by mass.

“ A chemical compounds always found to be made up of same elements chemically combine togetherin the same fixed proportion by mass”.

Example of lemon juice:

In this example we have to make double or triple amount

Of lemon juice, all we have to use the proportionate amount

Of the individual ingredients i.e (sugar, lemon, water). Water + Sugar + lemon

Proportionate Amount: lemonade

It means that we have to make the drink

for ten people instead of five , we just have  to take double

Amount of ingredients anything more or less would result in change in taste.

Similarly in any chemical substance the elements present are always in fixed proportion by mass so that’s why its is also called law of definite proportion.

Let us take the example of elements A and B giving the compound C:

In this reaction we can notice that it has two atom of element A and three atoms of element B.

Is it true for all the unit making up the compound C ? “Yes”

It means that even if one atom From any of these unit is removed or added that it can no longer be called compound C , it will change to something.

So it number of atom is fixed then the mass will be also constant

Example of ammonia :

If we decomposed ammonia by any means say by extreme heat or some other way, then we get the nitrogen and hydrogen atoms in the same proportions by mass.

If we take 17gm of ammonia and decomposed it then we will get 14gm Nitrogen and 3gm Hydrogen. So we can say Nitrogen and Hydrogen are present in ammonia , in a constant ratio 14:3.

If we decomposed ammonia in other way the ratio of Nitrogen and Hydrogen in the compound will always be the same 14:3.

Example in water molecule:

In the water molecule 2 atom of hydrogen and one atom of oxygen are present

So ratio of water molecule (H2O)is calculate by following procedure

2×H/O.

.                                =2×1/16

=2/16

=1/8

So by the above procedure the ratio of water molecule is 1:8

Question Answer:

1) What is law of constant proportion?

Answer: “ A chemical compounds always found to be made up of same elements chemically combine together in the same fixed proportion by mass”.

2) Write the statement of law of constant proportion?

Answer: : In a chemical substance elements are always in fixed proportion by mass.

3) Who proposed law of constant proportion?

Answer: The law of constant proportion was proposed by a French chemist joseph L. Proust. In 797.

4) Give the example of law of constant proportion?

Answer: Example in water molecule:

In the water molecule 2 atom of hydrogen and one atom of oxygen are present

So ratio of water molecule (H2O)is calculate by following procedure

2×H/O.

.                                =2×1/16

=2/16

=1/8

So by the above procedure the ratio of water molecule is 1:8

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