Empirical Formula
—
Simplest whole-number ratio
Molar Mass of Empirical Formula
—
g/mol
Find the empirical formula (simplest whole-number ratio) of chemical compounds from mass composition, percentage composition, or a known molecular formula. Step-by-step solutions included.
Problem: A 25.0 g sample of a compound contains 8.34 g of carbon (C) and 16.66 g of oxygen (O). Find the empirical formula.
Solution:
Moles of C = 8.34 / 12.011 = 0.694 mol
Moles of O = 16.66 / 15.999 = 1.041 mol
Ratio: C = 0.694/0.694 = 1, O = 1.041/0.694 = 1.5
Multiply by 2: C = 2, O = 3
Empirical Formula: C₂O₃
Problem: A compound contains 40.00% carbon (C), 6.71% hydrogen (H), and 53.29% oxygen (O) by mass. Find the empirical formula.
Solution:
Assume 100 g: 40.00 g C, 6.71 g H, 53.29 g O
Moles: C = 40.00/12.011 = 3.33 mol, H = 6.71/1.008 = 6.66 mol, O = 53.29/15.999 = 3.33 mol
Ratio: C = 3.33/3.33 = 1, H = 6.66/3.33 = 2, O = 3.33/3.33 = 1
Empirical Formula: CH₂O (formaldehyde)
Problem: Find the empirical formula of glucose, whose molecular formula is C₆H₁₂O₆.
Solution:
Find the GCD of subscripts: C=6, H=12, O=6
Divide each by the GCD (6): C₆/₆H₁₂/₆O₆/₆
Empirical Formula: CH₂O
Problem: Ascorbic acid has the molecular formula C₆H₈O₆. What is its empirical formula?
Solution:
Find GCD of subscripts: C=6, H=8, O=6
The GCD of 6 and 8 is 2. Divide: C₆/₂H₈/₂O₆/₂
Empirical Formula: C₃H₄O₃
The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound. It is determined by converting mass data to moles, finding the simplest ratio, and scaling to whole numbers where needed.
Divide the mass (in grams) of each element by its atomic mass. For percentage composition, assume a 100 g sample — then percentages become grams directly.
Divide each mole value by the smallest mole value among all elements. This gives the mole ratio.
If any ratio is within 0.05 of a whole number, round it. Otherwise, multiply all ratios by a small integer (2, 3, or 4) to get whole numbers.
Write the element symbols followed by the whole-number subscripts. If the subscript is 1, it is omitted. The result is the empirical formula.
1.008 g/mol — the lightest element, found in water, acids, and organic compounds.
12.011 g/mol — the building block of all organic chemistry and life on Earth.
14.007 g/mol — essential for proteins, DNA, and many industrial compounds.
15.999 g/mol — abundant in the atmosphere, water, and countless compounds.
32.065 g/mol — found in amino acids and many minerals.
30.974 g/mol — key component of DNA, ATP, and bone tissue.
The empirical formula of a chemical compound is the simplest whole-number ratio of atoms of each element present in the compound. It represents the smallest possible integer ratio of elements that maintains the compound's composition.
For example, hydrogen peroxide has the molecular formula H₂O₂, which tells us that each molecule contains 2 hydrogen atoms and 2 oxygen atoms. The empirical formula is HO, because the ratio of H:O is 1:1 (the simplest whole-number ratio).
Different compounds can share the same empirical formula. For instance, glucose (C₆H₁₂O₆), formaldehyde (CH₂O), and acetic acid (C₂H₄O₂) all have the empirical formula CH₂O, even though they are very different substances with different molecular formulas and properties.
The molecular formula tells you the exact number of atoms of each element in one molecule of a compound. The empirical formula tells you the simplest ratio of those atoms.
| Compound | Molecular Formula | Empirical Formula | n |
|---|---|---|---|
| Water | H₂O | H₂O | 1 |
| Hydrogen Peroxide | H₂O₂ | HO | 2 |
| Glucose | C₆H₁₂O₆ | CH₂O | 6 |
| Acetic Acid | C₂H₄O₂ | CH₂O | 2 |
| Dinitrogen Tetroxide | N₂O₄ | NO₂ | 2 |
| Benzene | C₆H₆ | CH | 6 |
To find the molecular formula from the empirical formula, you need the molar mass of the compound. The ratio n = (molar mass of compound) / (molar mass of empirical formula) tells you how many empirical formula units make up one molecule.
⚠️ Important Note: Empirical formulas represent the simplest ratio of elements and may not reflect the actual molecular structure. To determine the molecular formula, you need additional information such as the molar mass of the compound. This calculator is for educational and reference purposes. Always verify results with experimental data for critical applications.