CBSE Class 11-science Answered

Law of Constant Composition-

• According to the Law of Constant Composition, in a chemical substance, elements are always present in a definite proportion by mass.
• For example, water obtained from any source will have the same two elements, namely hydrogen and oxygen present in it.
• 2 grams of hydrogen and 16 grams of oxygen form a molecule of water. The proportion of hydrogen and oxygen is 1 : 8 by mass. This proportion will always remain the same, irrespective of the source of water.
• Similarly, carbon dioxide obtained from any source will contain the same two elements, carbon and oxygen.
• 12 grams of carbon and 32 grams of oxygen form a molecule of carbon dioxide. Carbon dioxide obtained from any source will always have the proportion of masses of carbon and oxygen as 3 : 8.