A compound's empirical formula is the simplest whole number ratio of atoms in a compound, whereas the molecular formula is the actual number of atoms of each element present in a single molecule of a compound. The empirical formula can be the same as or a multiple of the molecular formula. In this article, we will study the difference between the Empirical formula and Molecular Formula.
Empirical Formula- Definition
The empirical formula represents the simplest whole-number ratio of atoms in a molecule of a compound. It represents the minimum ratio of elements present in the compound and is expressed as the smallest possible whole-number ratio of the atoms in a compound molecule. For example, the empirical formula of hydrogen peroxide is HO. It represents that hydrogen and oxygen (H: O) are present in the ratio of 1: 1. Similarly, benzene is CH which indicates that the atomic ratio of C: H in benzene is 1: 1.
1. Molecular Formula- Definition
The formula gives the actual number of atoms of each element present in one molecule of the compound. For example, since a molecule of hydrogen peroxide contains 2 atoms of hydrogen and 2 atoms of oxygen, the molecular formula is H2O2. Similarly, the molecular formula of benzene is C6H6, which indicates that one molecule of benzene contains 6 carbon atoms and 6 hydrogen atoms.
2. Relation between Empirical and Molecular Formula
Molecular formula = n (Empirical formula) Where n is a simple whole number. It is equal to
3. Empirical Formula Vs Molecular Formula
Empirical Formula | Molecular Formula |
It is the simplest whole number ratio of atoms in a molecule. | It is the exact number of atoms of each element in a molecule. |
It represents the smallest unit of a molecule. | It represents the actual arrangement of atoms in a molecule. |
It can be determined from chemical analysis. | It can be determined from spectroscopic data or X-ray diffraction. |
Can be used to calculate molecular formula. | Can be used to calculate the empirical formula. |
4. How to convert a molecular formula to its empirical formula:
To convert a molecular formula to its empirical formula, follow these steps:
Determine the molecular weight of the molecule by adding up the atomic weights of all the atoms in the molecular formula.
Divide each atomic weight in the molecular formula by the smallest common multiple of all the atomic weights.
Round the resulting atomic ratios to the nearest whole number.
Build the empirical formula from the atomic ratios obtained in the previous step.
5. How to find a molecular formula using its empirical formula
To find the molecular formula using an empirical formula, you need to determine the molecular weight of the molecule and then compare it to the molecular weight of the empirical formula. The molecular weight of the empirical formula can be calculated by multiplying the atomic weight of each element in the empirical formula by the number of times that element appears in the formula. Below are the steps involved:
Determine the molecular weight of the compound by using mass spectrometry or another appropriate method.
Calculate the molecular mass of the empirical formula by multiplying the atomic weight of each element in the formula by the number of times it appears.
Divide the molecular weight of the compound by the molecular weight of the empirical formula. The result is the ratio between the molecular formula and the empirical formula.
Multiply all the subscripts in the empirical formula by the ratio obtained in the previous step to get the molecular formula.
6. Sample Calculations Q. A
welding fuel gas contains carbon and hydrogen. Burning a small sample of it in oxygen gives 3.38g of carbon dioxide, 0.690 g of water, and no other products. A volume of 10.0 L of this welding gas is found to weigh 11.6 g. Calculate (i) Empirical formula (ii) Molar mass of the gas (iii) Molecular formula Solution: CO2 (44g) ≡ C (12g) Mass of carbon H2O (18g) ≡ H2 (2g) Mass of hydrogen = Percentage of C Percentage of H (i) Calculation of Empirical Formula
Element | % | Atomic Mass | Moles of Atom | Mole ratio | Simplest whole no. ratio |
C | 92.2 | 12 | 1 | ||
H | 7.77 | 1 | 1 Therefore, the empirical formula is CH. (ii) Calculation of molar mass 10.0 L of gas at S.T.P weighs = 11.6g 22.4 L of gas at S.T.P weighs (iii) Calculation of Molecular Formula Empirical formula mass = 12 + 1 =13 Molecular mass = 26 Molecular formula = 2(CH) = C |
Therefore, the empirical formula is CH. (ii) Calculation of molar mass 10.0 L of gas at S.T.P weighs = 11.6g 22.4 L of gas at S.T.P weighs (iii) Calculation of Molecular Formula Empirical formula mass = 12 + 1 =13 Molecular mass = 26 Molecular formula = 2(CH) = C2 H2. Q. An organic compound containing carbon, hydrogen, and oxygen gave the following percentage composition: C = 40.68%, H = 5.08% Calculate the molecular formula of the compound. Solution: (i) Calculation of empirical formula
Element | % | Atomic Mass | Moles of Atom | Mole ratio | Simplest whole no. ratio |
C | 40.68 | 12 | 2 | ||
H | 5.08 | 1 | 3 | ||
O | 100 – (40.68+5.08) =54.24 | 16 | 2 The empirical formula = C |
The empirical formula = C2 H3 O2 Empirical formula mass = 2 ×12 + 3 × 1 + 2 × 16 = 59. (ii) Calculation of Molecular formula Molecular formula = 2(C2 H3 O2) = C4 H6 O4. Also Refer: What Are The Different Ways To Represent Compounds?
Key Takeaways
A compound's empirical formula is the simplest whole number ratio of atoms in a compound.
The molecular formula is the actual number of atoms of each element present in a single molecule of a compound.
The empirical formula can be the same as or a multiple of the molecular formula.
FAQs
1. What is the difference between an empirical formula and a molecular formula?
The empirical formula represents the simplest whole-number ratio of atoms in a compound, while the molecular formula represents the actual number of atoms of each element in a molecule.
2. Can an empirical formula and a molecular formula be the same?
Yes, if the empirical formula represents the exact count of atoms in a molecule, it will be the same as the molecular formula.
3. How do I calculate the empirical formula?
aTo calculate the empirical formula, determine the number of moles of each element in a sample of the compound, then divide each mole value by the smallest mole value and round to the nearest whole number. The result is the empirical formula.
4. How do I calculate the molecular formula from the empirical formula?
To calculate the molecular formula from the empirical formula, multiply the subscripts of the empirical formula by the factor that makes the molecular weight equal to the measured molecular weight of the compound. The result is the molecular formula.
Conclusion
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