Empirical and Molecular Formulas
The empirical formula of a compound represents the simplest ratio of the number of atoms of each element in the compound. To determine the empirical formula, you can use the composition by mass of the compound. Here are the steps:
- Divide the mass (or percentage mass) of each element by its atomic mass (Ar).
- Use the results to calculate the simplest whole-number ratio of atoms in the compound.
Example
Let’s say we have a compound that is 40% by mass carbon and 60% by mass oxygen. To determine the empirical formula, we follow these steps:
Step-by-step Solution:
| Element | % by Mass (m) | Atomic Mass (Ar) | m / Ar | Moles | Ratio of Moles |
|---|---|---|---|---|---|
| C | 40.0 | 12 | 40.0/12 | 3.33 | 2 |
| O | 60.0 | 16 | 60.0/16 | 3.75 | 3 |
Result: The ratio of moles is approximately 2:3, so the empirical formula of the compound is C₂O₃.
Practice Problem
Now, try the following example to determine the empirical formulas:
(a) Compound X — composition by mass: 68.4% nitrogen, 31.6% hydrogen
(b) Compound Y — composition by mass: 52.3% sulfur, 47.7% oxygen
Molecular Formula
In contrast to the empirical formula, the molecular formula shows the actual number of atoms of each element in the compound. It represents a specific molecule, while the empirical formula provides only the simplest ratio. The molecular formula is always a multiple of the empirical formula.
Example
Consider a compound with the empirical formula CH₃, and its molar mass is 30. We can find the molecular formula as follows:
- Calculate the formula mass of the empirical formula:
(1 × 12) + (3 × 1) = 15 - Since the molar mass is 30, and 30 ÷ 15 = 2, the molecular formula must be C₂H₆.
Thus, the molecular formula is C₂H₆, which is twice the empirical formula CH₃.
Writing and Balancing Chemical Equations
Chemical equations are a shorthand way of representing chemical reactions. The symbols for elements from the periodic table make these equations universally understandable. When writing a chemical equation, you should follow these basic steps:
- Check the formula of each compound: Ensure that you are using the correct chemical formulas for all substances involved.
- Balance the equation: Make sure that the number of atoms of each element is the same on both sides of the equation.
- Consider the physical states: Visualize the reaction and use the appropriate state symbols:
- (s) for solid,
- (l) for liquid,
- (g) for gas,
- (aq) for aqueous solution (a substance dissolved in water).
Following these steps ensures that chemical equations accurately reflect real-world reactions.
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