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Chemistry is governed by fundamental laws that describe how substances combine and react. These laws form the foundation for understanding chemical reactions and the nature of matter.
This law states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
Mathematically,
\[ \text{Mass of reactants} = \text{Mass of products} \]Example: When hydrogen reacts with oxygen to form water, the combined mass of hydrogen and oxygen before reaction equals the mass of water formed.
This law states that a chemical compound always contains the same elements in the fixed ratio by mass, regardless of the source or method of preparation.
For example, water (H\(_2\)O) always contains hydrogen and oxygen in the mass ratio of approximately 1:8.
If a compound contains elements A and B, then the ratio of masses of A to B is constant:
\[ \frac{\text{Mass of A}}{\text{Mass of B}} = \text{constant} \]When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.
Example: Carbon and oxygen form two compounds — carbon monoxide (CO) and carbon dioxide (CO\(_2\)).
The ratio of oxygen masses combining with fixed carbon mass is:
\[ \frac{32}{16} = 2 \]This simple whole number ratio confirms the law.
---John Dalton, in the early 19th century, proposed a theory to explain the laws of chemical combination based on the concept of atoms.
Dalton introduced the concept of atomic mass as the relative mass of an atom compared to hydrogen, which he assigned a mass of 1.
Modern atomic theory defines the atomic number (\(Z\)) as the number of protons in the nucleus, which determines the element's identity.
Significance:
Limitations:
Using the laws and atomic theory, chemists can predict the amounts of substances consumed and produced in reactions.
Example: Consider the reaction of hydrogen and oxygen to form water:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]From the balanced equation, 4 g of hydrogen reacts with 32 g of oxygen to produce 36 g of water, illustrating the law of conservation of mass.
The law of definite proportions helps determine molecular formulas by analyzing the mass percentages of elements in a compound.
Example: A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Using atomic masses, the empirical formula can be calculated.
When 10 g of hydrogen reacts with 80 g of oxygen, what is the mass of water formed?
Solution:
According to the law of conservation of mass,
\[ \text{Mass of water} = \text{Mass of hydrogen} + \text{Mass of oxygen} \] \[ = 10,g + 80,g = 90,g \]Answer: 90 g of water is formed.
A compound contains 24 g of carbon and 32 g of oxygen. Find the mass ratio of carbon to oxygen and verify if it follows the law of definite proportions.
Solution:
\[ \text{Mass ratio} = \frac{24}{32} = \frac{3}{4} = 0.75 \]Since this ratio is fixed for this compound, it follows the law of definite proportions.
Answer: Mass ratio of carbon to oxygen is 3:4.
Two oxides of nitrogen contain nitrogen and oxygen in the following mass ratios:
Verify the law of multiple proportions.
Solution:
\[ \text{Ratio of oxygen masses} = \frac{32}{16} = 2 \]This is a simple whole number ratio, confirming the law.
Answer: The law of multiple proportions is verified.
A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Calculate its empirical formula.
Solution:
Assume 100 g of compound:
Calculate moles of each element:
\[ \text{Moles of C} = \frac{40}{12} = 3.33 \quad \text{Moles of H} = \frac{6.7}{1} = 6.7 \quad \text{Moles of O} = \frac{53.3}{16} = 3.33 \]Divide by smallest number of moles (3.33):
\[ C = 1, \quad H = 2, \quad O = 1 \]Empirical formula is \( CH_2O \).
Answer: Empirical formula is \( CH_2O \).
According to Dalton's theory, if 2 atoms of element A combine with 3 atoms of element B to form a compound, what is the simplest whole number ratio of atoms in the compound?
Solution:
The ratio is given directly as 2:3.
Answer: The simplest whole number ratio is 2 atoms of A to 3 atoms of B.
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