About the Law of Multiple Proportions
The British scientist John Dalton (1766-1826) studied different binary compounds and developed the Law of Multiple Proportions. What are compounds? In chemistry a compound is a substance that is formed when there is a chemical combination of elements in fixed proportions. So then a binary compound is simply a compound that consists of two elements.
What Dalton found was that when there are two elements, A and B, the mass (quantity of matter in an object) of element B in combination with a fixed mass of A results in a whole amount that can be shown in a ratio that is a small whole number. This foundation to chemistry is known as the law of multiple proportions.
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The Facts
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Dalton's Atomic Theory of 1805 explained what experiments had shown. He determined laws for chemical combination in terms of an atomic theory of matter. This was the first major atomic theory. It was important in the development of chemistry.
The Facts
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At times an element can combine with another element to form two or more different compounds. An example is when the elements carbon and oxygen form two different compounds: carbon monoxide and carbon dioxide.
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The Facts
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Example: Make two oxide compounds using a fixed mass amount of carbon (C). Use two different mass amounts of oxygen (O). The mass of the carbon is 2.4g.
The mass of the oxygen in the first compound is 3.2.g; in the second, it is 6.4g. Here are the chemical statements showing each.
2.4 g C +6.4 g O = 8.8 g oxide
2.4 g C+ 3.2 g O = 5.6 g oxide
One oxide is carbon monoxide, with the formula CO. The other has two times the number of oxygen and is carbon dioxide, CO2. The ratios are 3.2:2.4 and 6.4:2.4.
The Facts
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Applying the law of multiple proportions you see that when two elements (A and B) combine to form more than one compound, the different masses of A that combine with a fixed mass of B are in a simple ratio for the 'law of multiple proportions.
In the above example, the element A is represented by the two different masses 3.2 g O and 6.4 g O. The element B, a fixed mass, is represented by the mass 2.4 g C.
The Facts
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Since the carbon remains a set amount this is the fixed mass of the law of multiple proportions. The oxygen amounts represent the different masses that combine with the carbon in a simple ratio. The result is a small whole number.
The masses of 6.4 g O and 3.2 g O when individually combined with the mass of 2.4 g C can be set up in a ratio with the result being a small whole number. So, in this case where you have 2.4 g C as the fixed amount then the ratio of the other components will be 6.4:3.2 or
6.4 g O divided by 3.2 g O = 2.0
Dalton's third postulate of his atomic theory in 1805 explains the law of multiple proportions.
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