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The laws of chemical combination describe the essential principles obeyed by the interacting atoms and molecules. The combination of elements to make compounds is governed by the five basic laws, which lay the groundwork for the analysis of chemical reactions. These laws provide a mathematical foundation and allow predictability under given initial conditions.
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Law of Definite Proportions
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The law of definite proportions was first observed by a French chemist known as Joseph Proust. This law states that a particular compound always consists of equal amounts of elements by weight. Proust worked with two samples of the cupric carbonate, among which one was of natural origin and the other one was synthetic. From this, he found that the composition of elements present in both the samples was the same as mentioned below:
| - | % of copper | % of oxygen | % of carbon |
|---|---|---|---|
| Natural sample | 51.35 | 9.74 | 38.91 |
| Synthetic sample | 51.35 | 9.74 | 38.91 |
Check Important Notes for Crystalline Solid
Thus, each compound has the same elements in the same proportions, regardless of where the compound was obtained or who made it, or its mass.
Example:
Let us take a compound of ethyl alcohol (C2H6O) which consists of 2 atoms of carbon,6 atoms of hydrogen, and one atom of oxygen.
| Atoms | Weight of the atoms |
|---|---|
| 2 atoms of C | 2*12g/mol =24 |
| 6 atoms of H | 6*1g/mol=6 |
| 1 atom of O | 1*16g/mol=16 |
| Total weight of C2H6O | 46g/mol |
Based on the law of definite proportions, the percentages of C, H, and O will be the same by weight no matter what the amount is.
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Law of Multiple Proportions
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This law was proposed by Dalton in the year 1803. According to this law, if 2 elements can combine to form quite one compound, the masses of 1 element that combine with a hard and fast mass of other elements are within the ratio of small whole numbers.The law is a rule of stoichiometry. It is an extension of the law of definite proportions.
Example:
Hydrogen + oxygen = water
2g + 16g = 18 g
Hydrogen + oxygen = hydrogen peroxide
2g + 32g = 34g
Here the masses of oxygen which combines with the same mass of hydrogen makes a ratio of 1:2
There is a certain limitation of this law that is best suited for only simple compounds. eg: if we take hydrocarbons such as decane(C10H22), one would find that 10 grams of C would react with 18.6 grams of H, for the ratio of hydrogen masses about 121:120, making a hard ratio. It also doesn't work with polymers and oligomers.
Law of Conservation of Mass
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This law was proposed by Lavoisier. The law of mass conservation states that the mass that exists in an isolated system can neither be generated nor destroyed by any kind of chemical reactions or physical transformations.
After performing several careful combustion reactions, he concluded that states that despite chemical reactions or physical transformations, mass is conserved
In other words, the mass of any particular element at the starting of a reaction will equal the mass of that element at the end. If we account for all the reactants and products in a chemical reaction, the total mass will be the same at any point in time in a closed system. The law of conservation of mass is beneficial for a variety of calculations and to unravel the unknown masses within the reaction happening.
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Gay Lussac’s Law of Gaseous Volumes
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This law was stated by Gay lissac in 1808. He observed that when the gases are merged or are generated during a particular reaction, they do it with a simple ratio by volume with a condition provided that every gas is under the same temperature and pressure.
The mathematical formula of the law is as follows:
P ∝ T ; P/T = k
Where P stands for the pressure exerted by the gas
T is that the temperature of the gas
k is a constant
The graphical representation of the law is as follows:
Gay Lussac’s Law of Gaseous Volumes
Example:
Automobile tires: the pressure of tires drops in winters and soars in summer. if one fills more air in tires during winter, they could over pressurize when they heat up. During summer, the temperature of air increases, thus pressures of air increase which increases the chances of fracturing of tires.
Read More: Conservation Laws in Physics
Avogadro Law
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It is the equation proposed by Avogadro which states that under an equivalent condition of temperature and pressure, equal volumes of all the gases contain an equivalent number of molecules.
The mathematical representation for this gas law is as follows:
k = V/n
Where k is proportionality constant, V is the volume of a gas and n is the number of moles of a gas. thus,
V1/n1 = V2/n2
There is a limitation of this law that even though it is perfectly applicable to ideal gases, Avogadro’s law provides a relationship for only real gases. The deviation of the important gases from the perfect behavior tends to extend at higher pressures and lower temperatures. Gaseous molecules that have lower molecular masses like hydrogen follow Avogadro’s law to an honest extent compared to the heavier molecules.
Things to Remember
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- Laws of chemical combinations is a part of first term chemistry syllabus unit 1 Some Basic Concepts of Chemistry.
- It carries a total of 10 periods and 4 to 6 marks.
- Law of definite proportions states that each chemical compound has a definite and fixed amount of mass in its basic elements.
- Law of multiple proportions states that when there is a combination between two elements in order to form more than one compound the weight of a single element merging with a definite weight of the other forms a ratio of small whole numbers.
- Avogadro law formula is V1/n11 = V2/n2
- The mathematical formula of the Gay Lussac’s law is P ∝ T ; P/T = k
Read Further: Physical and Chemical Classification of Matter
Sample Questions
Ques 1. Which of the following pairs of substances illustrate the law of multiple proportions. (1 mark)
(i) CO and CO2
(ii) H2O and D2O
(iii) NaCl and NaBr
(iv) MgO and Mg(OH)2
Ans: A (here C is the common element and O is in 1:2 ratio, rets all other options have different elements)
Ques 2. The percentage of copper and oxygen in samples of CuO obtained by different methods were the same.is illustrated which law? (1 mark)
(i) Constant proportion
(ii) Conservation of mass
(iii) Multiple proportions
(iv) Reciprocal proportions
Ans: C
Ques 3. Chemical equations are balanced according to which law? (1 mark)
(i) Multiple proportions
(ii) Reciprocal proportion
(iii) Conservation of mass
(iv) Definite proportions
Ans: C
Ques 4. The law of multiple proportions is illustrated by two compounds? (1 mark)
(i) NaCl and NaBr
(ii) Ordinary water and heavy water
(iii) Caustic soda and caustic potash
(iv) Sulphur dioxide and sulphur trioxide
Ans: Option C as the law of multiple proportions talks about the ratios of two compounds consisting of the same elements in integral values.
Ques 5. Two elements X and Y have atomic weights of 14 and 16. They form a series of compounds A, B, C, D, and E in which the same amount of element X, Y is present in the ratio 1: 2 : 3: 4: 5. If compound A has 28 parts by weight of X and 16 parts by weight of Y, then the compound of C will have 28 parts weight of X and y. (1 mark)
(i) 32 parts of y
(ii) 48 parts of y
(iii) 64 parts of y
(iv) 16 parts of y
Ans: Given, x=14, y=16.
According to the given data, the compounds formed by x and y will be, XY,xy2,xy3,xy4, and xy5. In this case, the ratio of x:y is 28:16, so in xy3, the ratio will be 28:48.
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