Empirical and Molecular Formula: Definition, Questions and Examples

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:39 PM IST

The chemical formula may be of two types- empirical formula and molecular formula. The formula which gives the simplest whole number ratio of the atoms of various elements present in one molecule of the compound is called the empirical formula whereas the formula which gives the actual number of atoms of various elements present in one molecule of the compound is called molecular formula.

This Story also Contains

What is a Chemical Formula?
Empirical Formula
Molecular Formula
Some Solved Example
Summary

Empirical and Molecular Formula: Definition, Questions and Examples

What is a Chemical Formula?

A chemical formula represents the combination of atoms of all the elements that make up a compound. It represents the relative ratio of atoms of its constituent elements. In the case of a compound, it represents one molecule, one mole, and one gram molecular weight of the compound.
For example, CuSO₄.5H₂O represents one molecule, one mole, and one gram molecular weight of hydrated copper sulfate.

Empirical Formula

It is the simplest ratio of the number of atoms of different elements present in one molecule of a compound. It does not represent the actual number of atoms of different elements present in one molecule of the compound.

How to find out the empirical formula and the molecular formula in case the % composition of the compound is given to us

Step 1. Conversion of a mass percent to grams.

Step 2. Convert grams into the number of moles of each element.

Step 3. Divide the mole value obtained above by the smallest number.

Step 4. Write the empirical formula by mentioning the numbers obtained above after writing the symbols of respective elements.

Step 5. Write molecular formula (Molecular formula) with the help of the information given.

Molecular formula is a whole number multiple of the empirical formula

Molecular formula $=(\text { Empirical formula })_{\mathrm{n}}$

where n is the whole number.

Molecular Formula

It shows the actual number of atoms of different elements present in one molecule of a compound.

n = (Molecular weight) / (Empirical formula weight)
Molecular weight can be directly given or some other information like Vapour density can be given which will enable us to calculate the molecular weight.
Molecular weight = 2 x Vapour Density
For some compounds, the molecular formula and empirical formula may be the same also.

Some Solved Example

Que 1. A 5.325 g sample of Methyl Benzoate, a compound used in the manufacturing of perfumes, is found to contain 3.758g of Carbon, 0.316g of Hydrogen and 1.251g of Oxygen. What is the empirical formula of the compound?

1) C₄H₄O

2) C₂H₄O

3) C₂H₂O

4) C₄H₃O

Solution

For glucose, the empirical formula is CH₂O

Element	%	Mole ratio	Simplest mole ratio
C	$\frac{3.758 \times 100}{5.325}=70.57$	$\frac{70.57}{12}=5.88$	$\frac{5.88}{1.47}=4$
H	$\frac{0.316 \times 100}{5.325}=5.93$	$\frac{5.93}{1}=5.93$	$\frac{5.93}{1.47}=4$
O	$\frac{1.251 \times 100}{5.325}=23.50$	$\frac{23.50}{16}=1.47$	$\frac{1.47}{1.47}=1$

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So, the Empirical formula is C₄H₄O

Hence, the answer is an option (1).

Que 2: An organic compound containing C & H has 92.3% C. Its empirical formula is:

1) CH₂

2) CH₃

3) CH₄

4) CH

Solution

Element	%	Atomic Mass	Relative no. of atoms	Simplest ratio
C	92.30	12	7.69	1
H	7.70	1	7.70	1

So, its empirical formula is CH

Hence the answer is an option (4).

Que 3. The ratio of mass percent of C and H of an organic compound (C_XH_YO_Z) is 6 : 1. If one molecule of the above compound (C_XH_YO_Z) contains half as much oxygen as required to burn one molecule of compound C_XH_Y completely to CO₂ and H₂O. The empirical formula of compound C_XH_YO_Z is :

1) C₂H₄O₃
2) C₃H₆O₃
3) C₂H₄O
4) C₃H₄O₂

Solution

The mass ratio of C & H is 6 : 1

So mole ratio of C & H is 1 : 2

$\therefore$ X : Y = 1 : 2

To burn one molecule of $C_X H_Y$

$\mathrm{CH}_2+\frac{3}{2} \mathrm{O}_2 \rightarrow \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}$

$\frac{3}{2}$ molecule of O₂ is required i.e. 3 atoms of O are required so $X: Y: Z=1: 2: \frac{3}{2} \Rightarrow X: Y: Z=2: 4: 3$

So empirical formula is $\mathrm{C}_2 \mathrm{H}_4 \mathrm{O}_3$

Hence, the answer is an option (1).

Que 4. A compound has an empirical formula C₂H₄O. An independent analysis gave a value of 132 for its molecular mass. What is the correct molecular formula?

1) C₄H₄O₅

2) C₁₀H₁₂

3) C₆H₁₂O₃

4) C₄H₈O₅

Solution

$\mathrm{n}=\frac{\text { Molecular Mass }}{\text { Empirical Formula mass }}=\frac{132}{44}=3$

Therefore, Molecular formula is (C₂H₄O)₃ = (C₆H₁₂O₃)

Hence, the answer is an option (3).

Que 5. An organic compound contains 49.3% C, 6.84% H and the remaining is oxygen and its vapor density is 73. The molecular formula of the compound is

1) C₃H₈O₂

2) C₆H₉O

3) C₄H₁₀O₂

4) C₆H₁₀O₄

Solution

Molecular mass $=2 \times 73=146$

% of oxygen = 100 - (49.3 + 6.84)
= 43.86%

Thus we have,

$C=\frac{\%}{100} \times \frac{\text { Molecular mass }}{\text { Atomic mass }}=\frac{49.3}{100} \times \frac{146}{12}=6$

$H=\frac{\%}{100} \times \frac{\text { Molecular mass }}{\text { Atomic mass }}=\frac{6.84}{100} \times \frac{146}{1}=10$

$O=\frac{\%}{100} \times \frac{\text { Molecular mass }}{\text { Atomic mass }}=\frac{43.86}{100} \times \frac{146}{16}=4$

Thus, Molecular Formula = C₆H₁₀O₄

Hence, the answer is an option (4).

Que 6. A compound on analysis was found to have the following composition: (i) Sodium 14.31%, (ii) Sulphur 9.97%, (iii) Oxygen 69.5%, (iv) Hydrogen 6.22%. Calculate the molecular formula of the compound assuming that the whole of Hydrogen in the compound is present as water of Crystallization. The molecular mass of the compound is 322.

1) Na₂SH₂₀O₁₀

2) Na₂SH₂₀O₁₄

3) Na₂SH₂₀O₇

4) Na₂SH₃₀O₁₆

Solution

Element	%age	Atomic mass	Relative no of atoms	Simplest Ratio
Na	14.31	23	0.622	$\frac{0.622}{0.311}=2$
S	9.97	32	0.311	$\frac{0.311}{0.311}=1$
H	6.22	1	6.22	$\frac{6.22}{0.311}=20$
O	69.50	16	4.34	$\frac{4.34}{0.311}=14$

Empirical formula = Na₂SH₂₀O₁₄

Empirical formula mass $=2 \times 23+32+20 \times 1+14 \times 16=322$

Molecular mass = 322

Molecular Formula = Na₂SH₂₀O₁₄

Hence, the answer is an option (2).

Summary

In a chemical formula, the actual number of atoms are represented in the compound. chemical formula shows the exact number of atoms present in it and their composition and the ratio of each element in a molecule. and the empirical formula represents the simple whole number of the ratio of atoms inside the compound. it show the number of atoms but not in exact quantity in the relative proportion.

Frequently Asked Questions (FAQs)

1. How can you determine the empirical formula of a compound?

To determine the empirical formula: 1) Find the mass of each element in the compound. 2) Convert masses to moles. 3) Divide each mole value by the smallest mole value. 4) If needed, multiply all ratios by a small whole number to get the simplest whole number ratio.

2. How does the concept of empirical formula relate to percentage composition?

The empirical formula is directly related to percentage composition. You can calculate the empirical formula from percentage composition by assuming 100g of the compound and converting the percentages to masses, then to moles, and finally to the simplest whole number ratio.

3. How does the concept of empirical formula apply to hydrates?

For hydrates, the empirical formula includes the water of crystallization. For example, the empirical formula of copper(II) sulfate pentahydrate is CuSO4•5H2O, showing the ratio of copper sulfate to water molecules in the crystal structure.

4. How can you use empirical formulas to predict chemical reactions?

Empirical formulas can be used in stoichiometry calculations to predict the amounts of reactants needed or products formed in a chemical reaction. They provide the simplest ratio of reacting elements, which can be scaled up as needed.

5. Why might a chemist prefer to use empirical formulas over molecular formulas in some situations?

Chemists might prefer empirical formulas when dealing with ionic compounds, which don't have discrete molecules, or when working with large polymers where the exact molecular mass is unknown. Empirical formulas are also useful for quick comparisons of composition.

6. What is the difference between an empirical formula and a molecular formula?

An empirical formula shows the simplest whole number ratio of atoms in a compound, while a molecular formula shows the actual number of atoms of each element in a molecule. For example, the empirical formula of glucose is CH2O, but its molecular formula is C6H12O6.

7. Why do we need both empirical and molecular formulas?

We need both because they provide different information. Empirical formulas are useful for quick comparisons and calculations, while molecular formulas give the exact composition of a molecule. Some compounds, like benzene (C6H6), have the same empirical and molecular formula.

8. What information do you need to convert an empirical formula to a molecular formula?

To convert an empirical formula to a molecular formula, you need the empirical formula and the molar mass of the compound. Divide the molar mass by the empirical formula mass to find the factor by which to multiply the empirical formula.

9. What role does the law of constant composition play in empirical and molecular formulas?

The law of constant composition states that a pure compound always contains the same elements in the same proportion by mass, regardless of its source. This principle is the foundation for empirical and molecular formulas, as they represent these fixed proportions.

10. Can different compounds have the same empirical formula?

Yes, different compounds can have the same empirical formula. For example, acetylene (C2H2) and benzene (C6H6) both have the empirical formula CH, but they are very different molecules with distinct properties.

11. How do empirical and molecular formulas relate to structural formulas?

Empirical and molecular formulas show the types and numbers of atoms in a compound, while structural formulas show how these atoms are arranged. You can derive empirical and molecular formulas from a structural formula, but not vice versa.

12. How does the empirical formula of a compound relate to its structure?

The empirical formula provides information about the relative numbers of atoms of each element in a compound, but it doesn't give information about the arrangement of these atoms or the actual molecular structure.

13. What is the significance of the subscript "x" in molecular formulas like (C6H10O5)x?

The subscript "x" in a molecular formula indicates that the molecule is a polymer, composed of repeating units. The value of "x" represents the number of repeating units in the polymer chain, which can vary.

14. How does the empirical formula relate to the concept of a chemical formula unit?

The empirical formula often represents a formula unit, especially for ionic compounds. A formula unit is the simplest electrically neutral grouping of ions in an ionic compound, which corresponds to the empirical formula.

15. Can a compound have more than one correct empirical formula?

No, a compound can only have one correct empirical formula. The empirical formula represents the simplest whole number ratio of atoms in a compound, which is a unique characteristic of that compound.

16. Can you have a fractional subscript in an empirical formula?

No, empirical formulas always use whole number subscripts. If calculations result in fractional subscripts, you should multiply all subscripts by the smallest whole number that will eliminate fractions.

17. How can you determine if a given formula is empirical or molecular?

You can't always tell just by looking, but generally, if the formula represents the simplest whole number ratio of atoms, it's empirical. If it represents the actual number of atoms in a molecule, it's molecular. Some formulas, like NaCl, are both empirical and molecular.

18. Why is it important to reduce empirical formulas to their simplest form?

Reducing empirical formulas to their simplest form ensures consistency and makes it easier to compare different compounds. It also simplifies calculations and helps in identifying the fundamental composition of a substance.

19. Why might the empirical formula of a compound be more useful in certain analytical techniques?

The empirical formula can be more useful in techniques like elemental analysis, where you determine the relative amounts of elements in a sample. It's also helpful in mass spectrometry, where you might only be able to determine the ratio of elements rather than the full molecular composition.

20. How does the concept of empirical formula apply to metals and alloys?

For metals and alloys, empirical formulas can represent the simplest whole number ratio of atoms in the crystal structure. For example, brass might have an empirical formula of Cu3Zn, representing the ratio of copper to zinc atoms.

21. Can you have an empirical formula with only one element?

Yes, elements that exist as molecules, like O2 or P4, can have empirical formulas. For example, the empirical formula of ozone (O3) is simply O, while its molecular formula is O3.

22. How does the empirical formula relate to the concept of stoichiometry?

Empirical formulas are crucial in stoichiometry as they represent the simplest whole number ratio of elements in a compound. This ratio is used to balance chemical equations and calculate the quantities of reactants and products in chemical reactions.

23. Why might a molecular formula be more useful than an empirical formula in organic chemistry?

In organic chemistry, molecular formulas are often more useful because they give the actual number of atoms in a molecule. This is important for understanding molecular structure, predicting reactions, and calculating molecular mass, which are all crucial in organic synthesis and analysis.

24. How can you use molar mass to distinguish between empirical and molecular formulas?

If the molar mass of a compound is equal to the mass represented by its formula, then the formula is molecular. If the molar mass is a multiple of the mass represented by the formula, then the formula is empirical, and you can determine the molecular formula by finding this multiple.

25. What role do empirical and molecular formulas play in the periodic table?

Empirical and molecular formulas use the symbols from the periodic table to represent elements. The periodic table's organization helps predict the types of compounds elements will form and their likely empirical formulas based on valence electrons and periodic trends.

26. How do empirical and molecular formulas relate to the concept of moles?

Both empirical and molecular formulas represent mole ratios of elements in a compound. The empirical formula represents the simplest whole number mole ratio, while the molecular formula represents the actual mole ratio in a molecule.

27. Can you have an empirical formula with a decimal subscript?

No, empirical formulas always use whole number subscripts. If calculations result in decimal subscripts, you should multiply all subscripts by the smallest whole number that will eliminate decimals.

28. How does the empirical formula relate to the law of multiple proportions?

The law of multiple proportions states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element are small whole numbers. Empirical formulas directly represent these ratios.

29. Why is it important to distinguish between empirical and molecular formulas in chemical nomenclature?

Distinguishing between empirical and molecular formulas is crucial in naming compounds correctly. Many ionic compounds are named based on their empirical formula, while molecular compounds are often named based on their molecular formula.

30. How can you use empirical formulas to predict the type of bonding in a compound?

Empirical formulas can give clues about bonding. For example, compounds with empirical formulas typical of ionic compounds (like NaCl) likely have ionic bonding, while those with empirical formulas typical of covalent compounds (like CH4) likely have covalent bonding.

31. What is the relationship between empirical formulas and crystal structures?

Empirical formulas often represent the simplest repeating unit in a crystal structure. For example, the empirical formula NaCl represents the simplest ratio of sodium to chloride ions in a sodium chloride crystal.

32. How do empirical and molecular formulas relate to the concept of isomers?

Isomers are compounds with the same molecular formula but different structural arrangements. They always have the same molecular formula, but may or may not have the same empirical formula, depending on whether the molecular formula can be reduced.

33. Can you determine the empirical formula of a compound from its mass spectrum?

Yes, mass spectrometry can be used to determine empirical formulas. The mass spectrum provides information about the mass-to-charge ratios of ions, which can be used to deduce the relative abundances of elements in the compound.

34. How do empirical and molecular formulas relate to the concept of oxidation states?

Empirical and molecular formulas can help in determining oxidation states of elements in a compound. By considering the overall charge and the typical oxidation states of other elements present, you can often deduce the oxidation state of a particular element.

35. Why might the empirical formula of a compound change under different conditions?

The empirical formula of a compound doesn't change under different conditions for a pure substance. However, if a compound can exist in different forms (like hydrates with varying amounts of water), the empirical formula might appear to change as conditions change.

36. How do empirical and molecular formulas relate to the concept of molecular geometry?

While empirical and molecular formulas provide information about the types and numbers of atoms in a compound, they don't directly indicate molecular geometry. However, molecular formulas can be used with other principles (like VSEPR theory) to predict geometry.

37. Can you have a compound where the empirical formula is the same as the molecular formula, but different from the structural formula?

Yes, this is possible. For example, glucose and fructose both have the molecular and empirical formula C6H12O6, but they have different structural formulas due to the different arrangement of atoms.

38. How do empirical and molecular formulas relate to the concept of percent yield in chemical reactions?

Empirical and molecular formulas are crucial for calculating theoretical yields in chemical reactions, which are then used to determine percent yields. The formulas allow you to determine the stoichiometric amounts of reactants and products.

39. Why is it important to consider empirical formulas when balancing chemical equations?

Empirical formulas represent the simplest whole number ratio of elements in a compound, which is crucial for balancing chemical equations. They ensure that the law of conservation of mass is obeyed in the balanced equation.

40. How can you use empirical formulas to predict the products of a chemical reaction?

Empirical formulas can help predict products by indicating the likely ratios of elements in the products. For example, if you react a metal with oxygen, you can often predict the empirical formula of the oxide based on the metal's common oxidation states.

41. What role do empirical and molecular formulas play in understanding chemical equilibrium?

Empirical and molecular formulas are essential for writing balanced equations for equilibrium reactions. They're used to express equilibrium constants and to perform calculations related to equilibrium concentrations.

42. How do empirical and molecular formulas relate to the concept of limiting reagents?

Empirical and molecular formulas are crucial for determining limiting reagents in chemical reactions. They allow you to calculate the molar ratios of reactants and products, which is necessary to identify which reagent will be completely consumed first.

43. Can you have a compound where the molecular formula is a fraction of the empirical formula?

No, this is not possible. The molecular formula is always a whole number multiple of the empirical formula. It can be the same as the empirical formula, or a multiple of it, but never a fraction of it.

44. How do empirical and molecular formulas relate to the concept of molarity in solutions?

Empirical and molecular formulas are used to calculate the molar mass of a compound, which is essential for preparing solutions of known molarity. The formulas allow you to convert between mass and moles of solute.

45. Why is it important to consider both empirical and molecular formulas when studying polymers?

For polymers, the empirical formula represents the simplest repeating unit, while the molecular formula represents the entire polymer molecule. Both are important: the empirical formula for understanding the basic composition, and the molecular formula for properties that depend on chain length.

46. How can empirical formulas be used to predict the products of combustion reactions?

Empirical formulas can help predict combustion products by showing the elements present in the compound. Generally, carbon-containing compounds produce CO2, hydrogen-containing compounds produce H2O, and other elements may form oxides or remain unchanged.

47. What role do empirical and molecular formulas play in understanding reaction mechanisms?

While empirical and molecular formulas don't directly show reaction mechanisms, they're crucial for writing balanced equations that represent each step in a mechanism. They ensure that atoms are conserved throughout the proposed mechanism.

48. How do empirical and molecular formulas relate to the concept of formula mass?

The formula mass is calculated using either the empirical or molecular formula, depending on the context. For ionic compounds, it's usually based on the empirical formula (formula unit). For molecular compounds, it's based on the molecular formula.

49. Can you use empirical formulas to distinguish between ionic and covalent compounds?

While not definitive, empirical formulas can often give clues about whether a compound is ionic or covalent. Compounds with empirical formulas typical of ionic compounds (like NaCl) are likely ionic, while those with formulas typical of molecules (like CH4) are likely covalent.

50. How do empirical and molecular formulas relate to the concept of structural isomers?

Structural isomers have the same molecular formula but different structural formulas. They may or may not have the same empirical formula, depending on whether their molecular formula can be reduced. Empirical and molecular formulas are crucial for identifying potential isomers.