6
6
12
6
180.156
g/mol
6
6
12
6
180.156
g/mol
The Molecular Formula Calculator converts an empirical formula into the actual molecular formula using experimentally determined molecular weight data. While the empirical formula gives the simplest atom ratio, the molecular formula reveals the true number of atoms in each molecule. This distinction is critical because many different compounds share the same empirical formula but have vastly different properties. For instance, formaldehyde (CH2O), acetic acid (C2H4O2), and glucose (C6H12O6) all reduce to the empirical formula CH2O. The molecular weight, typically obtained through mass spectrometry or colligative property measurements, provides the missing information needed to determine the correct molecular formula. This calculator divides the molecular weight by the empirical formula mass to find the integer multiplier, then scales all subscripts accordingly.
The molecular formula is found by determining the multiplier n:
n = MW / EFM
Where MW is the experimental molecular weight and EFM is the empirical formula mass. The value of n should be a positive integer (1, 2, 3, ...). Each subscript in the empirical formula is then multiplied by n to get the molecular formula subscripts.
For example, if the empirical formula is CH2O (EFM = 30.03 g/mol) and the molecular weight is 180.16 g/mol: n = 180.16 / 30.03 = 6.00 (rounds to 6). The molecular formula is C(1x6)H(2x6)O(1x6) = C6H12O6.
The verified molecular weight is calculated as EFM x n and should match the experimental MW within experimental error (typically within 1 g/mol). A significant mismatch suggests either an incorrect empirical formula or an inaccurate molecular weight measurement.
Common methods for determining molecular weight include: mass spectrometry (most accurate, identifies molecular ion peak), freezing point depression (cryoscopy), boiling point elevation (ebullioscopy), osmometry, and vapor density methods for gases.
The multiplier n tells you how many empirical formula units make up one molecule. If n = 1, the empirical and molecular formulas are identical. The molecular subscripts show the actual number of each type of atom in the molecule, which is essential for drawing structural formulas, predicting molecular geometry, and calculating molecular properties. The verified molecular weight should be very close to the experimental value; a discrepancy of more than 1-2% warrants re-examination of both the empirical formula and the molecular weight measurement.
Inputs
Results
With empirical formula CH2O (30.03 g/mol) and molecular weight 180.16 g/mol, the multiplier is exactly 6. This gives the molecular formula C6H12O6, which is glucose. The verified MW matches perfectly, confirming the calculation.
Inputs
Results
Benzene has the empirical formula CH (13.02 g/mol) and MW 78.11 g/mol. The multiplier of 6 gives C6H6, the famous aromatic hydrocarbon. Despite its simple empirical formula, benzene has a unique ring structure with delocalized electrons.
If the multiplier is not close to an integer (e.g., 2.7 or 3.4 instead of 3), this indicates an error in either the empirical formula or the molecular weight. Recheck the empirical formula derivation (especially the rounding of mole ratios) and verify the molecular weight measurement. Experimental errors of a few percent can cause non-integer multipliers.
Yes. When n = 1, the empirical and molecular formulas are identical. This is common for simple compounds like H2O, CO2, NH3, and many ionic compounds. About 30% of small molecules have n = 1.
In mass spectrometry, molecules are ionized and separated by mass-to-charge ratio. The molecular ion peak (M+) in electron ionization, or the [M+H]+ peak in electrospray, directly gives the molecular weight. High-resolution mass spectrometry can determine molecular weight to 0.001 amu, enough to determine the exact molecular formula unambiguously.
Colligative properties depend on the number of solute particles, not their identity. By measuring freezing point depression, boiling point elevation, or osmotic pressure of a solution with known concentration, the number of moles (and thus MW) can be calculated. For example, boiling point elevation: MW = (K_b x mass_solute) / (delta_T x mass_solvent).
Polymers consist of chains with varying lengths, so they have a distribution of molecular weights rather than a single value. They are characterized by number-average (M_n) and weight-average (M_w) molecular weights. The ratio M_w/M_n (polydispersity index) measures the breadth of the distribution. The concept of molecular formula applies to the repeating monomer unit instead.
For typical organic molecules, n ranges from 1 to about 10. However, for cyclic structures and macromolecules, n can be much larger. Cyclodextrins, for example, have empirical formulas like C6H10O5 with n = 6, 7, or 8 for alpha, beta, and gamma forms respectively, giving molecular formulas up to C48H80O40.
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