Beginner Topic 888 of 1102 Analytical, Lab & Spectroscopy
Molecular Formula
The actual number of each atom in a molecule, a multiple of the empirical formula.
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In-depth explanation of Molecular Formula
The molecular formula gives the actual number of atoms of each element in one molecule, and it is always a whole-number multiple of the empirical formula. The multiplier n equals the experimental molar mass divided by the empirical formula mass, and the molecular formula equals the empirical formula multiplied by n. Experimental molar mass is obtained from mass spectrometry, freezing point depression or vapour density measurements. Different compounds can share an empirical formula, such as ethyne and benzene for CH, and only the molar mass distinguishes them. Once the molecular formula is known, spectroscopic data are used to propose how the atoms are arranged.
Going deeper: Molecular Formula
Key ideas: Calculate n = molar mass/empirical formula mass and check that it is close to a whole number. Then multiply every subscript by n. Mass spectrometry often gives the molar mass from the molecular ion, and freezing point depression gives it from colligative data. Compounds such as C2H2 and C6H6 share an empirical formula of CH but differ in properties.
Common mistakes and connections: A typical error is using the molar mass of the compound in place of the empirical mass when computing n, or multiplying only one subscript by n. Students also assume that the empirical formula alone explains properties. Acetic acid, glucose and formaldehyde all have CH2O as empirical formula. Connects to mass spectrometry, isomerism and empirical formula determination.
Visualization of Molecular Formula
Whole-number multiples of the empirical formula CH2O have molar masses 30n, so a measured 180 g/mol gives n = 6 and the molecular formula C6H12O6. The interactive lesson shows this as a live animation or chart.
Worked example and use case
An empirical formula of CH2O (30 g/mol) with a molar mass of 180 g/mol gives n = 6, so the formula is C6H12O6.
Frequently asked questions about Molecular Formula
What is Molecular Formula?
The actual number of each atom in a molecule, a multiple of the empirical formula.
Is Molecular Formula a beginner, intermediate or advanced topic?
Molecular Formula is a beginner topic in Analytical, Lab & Spectroscopy. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Molecular Formula?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Density, Percent Composition, Empirical Formula.
What comes after Molecular Formula?
The next topics in Analytical, Lab & Spectroscopy are Balancing Equations and Stoichiometry.
Can I practise Molecular Formula with quizzes?
Yes. The interactive lesson includes a self-check question on Molecular Formula, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Molecular Formula fits in Analytical, Lab & Spectroscopy
Molecular Formula is topic 12 of 99 in Analytical, Lab & Spectroscopy.