Are elements that make up a compound always present in the same proportion?
What is meant by law of constant proportion and how is this law applied in experiment empirical formula of a compound?
The Law of Definite Proportions says that a given chemical compound always contains the same proportion by mass of its constituent elements. This is NOT the same as saying that elements always combine in a specific ratio, because they can combine in different ratios in different compounds. An example of this might be copper oxide which can be CuO or Cu2O, showing a different ratio of copper to oxygen. So, the answer to the question…
Does the properties of a compound often differ from the properties of the elements making up the compound?
No, the elements are not always in the same proportion. For example, two atoms of hydrogen combine with one atom of oxygen to make a single molecule of water. But one atom of hydrogen combines with one atom of fluorine to make a single molecule of hydrogen fluoride. The numbers of atoms involved depends on the number of electrons available in the outer electron clouds of the atoms involved.
The statement that a chemical compound always contains the same elements in exactly the same proportions is called the law of?
"In chemistry, the law of definite proportions and also the elements, sometimes called Proust's Law, states that a chemical compoundhttp://wiki.answers.com/wiki/Chemical_compound always contains exactly the same proportion of elementshttp://wiki.answers.com/wiki/Chemical_element by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition." - Wikipedia