How to Find Molecular Formula

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The molecular formula is an important piece of information for any chemical compound. The molecular formula tells you which atoms are present in the compound, and how many of each are present. You will need to know the empirical formula to calculate the molecular formula, and you will need to know that the difference between these two formulas is a whole number multiplier.

Part 1
Part 1 of 3:

Deriving the Molecular Formula from an Empirical Formula

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  1. The empirical formula provides the simplest, most reduced ratio of elements within a molecule, for example, two oxygens for every carbon. The molecular formula tells you how many of each of those atoms is present in the molecule. For example, one carbon and two oxygens (carbon dioxide). The two formulae are related by a whole number ratio such that if the empirical formula is multiplied by the ratio, it will yield the molecular formula. [1]
  2. This means utilizing the ideal gas law. You can determine the number of moles based on the pressure, volume, and temperature provided by the experimental data. The number of moles can be calculated using the following formula: n = PV/RT . [2]
    • In this formula, n is the number of moles, P is the pressure, V is the volume, T is the temperature in Kelvin, and R is the gas constant.
    • Example: n = PV/RT = (0.984 atm * 1 L) / (0.08206 L atm mol-1 K-1 * 318.15 K) = 0.0377 mol
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  3. This can only be done after finding the moles of gas present using the ideal gas law. You will also need to know how many grams of gas were present. Then divide the grams of gas by the moles of gas present to yield molecular weight.
    • Example: 14.42 g / 0.0377 mol = 382.49 g/mol
  4. Each atom in the empirical formula has its own atomic weight. This value can be found at the bottom of the atom’s square on the periodic table. Add these weights together to get the weight of the empirical formula. [3]
    • For example, carbon has an atomic weight of 12.0107, hydrogen has an atomic weight of 1.00794, and oxygen has an atomic weight of 15.9994. It's okay to look up the atomic weight if you don't know it.
    • Example: (12.0107 g * 12) + (15.9994 g * 1) + (1.00794 g * 30) = 144.1284 + 15.9994 + 30.2382 = 190.366 g
  5. In doing this, you can determine how many times the empirical weight is repeated within the actual molecule. Knowing how many times the empirical weight is repeated will let you find the number of times that the empirical formula repeats itself in the molecular formula. This should be a whole number. If the ratio is not a whole number, you will have to round it.
    • Example: 382.49 / 190.366 = 2.009
  6. Multiply the subscripts of the empirical formula by the ratio. This will yield the molecular formula. Note that for any compound with a ratio of “1,” the empirical formula and molecular formula will be the same.
    • Example: C12OH30 * 2 = C24O2H60
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Part 2
Part 2 of 3:

Finding the Empirical Formula

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  1. Sometimes the mass of each atom will be given. Other times, it will be given as a percent mass. If this is the case, assume a 100 g sample of the compound. This will allow you to write the percent mass as an actual mass in grams. [4]
    • Example: 75.46 g C, 8.43 g O, 16.11 g H
  2. You need to convert the molecular masses of each element to moles. In order to do this, you need to divide the molecular masses by the atomic masses of each respective element. You can find the atomic mass in the bottom of that element’s square on the periodic table. [5]
    • Example:
      • 75.46 g C * (1 mol / 12.0107 g) = 6.28 mol C
      • 8.43 g O * (1 mol / 15.9994 g) = 0.53 mol O
      • 16.11 g H * (1 mol / 1.00794) = 15.98 mol H
  3. You need to divide the number of moles for each separate element by the smallest molar amount from all the elements present in the compound. In doing so, you can find the simplest mole ratios. This works, because it gives sets the least abundant element to “1” and provides the respective ratios of other elements in the compound. [6]
    • Example: Smallest molar amount is oxygen with 0.53 mol.
      • 6.28 mol/0.53 mol = 11.83
      • 0.53 mol/0.53 mol = 1
      • 15.98 mol/0.53 mol= 30.15
  4. These numbers will become the subscripts in the empirical formula. You should round them to the nearest whole number. After finding these numbers, you can write the empirical formula. [7]
    • Example: The empirical formula would be C12OH30
      • 11.83 = 12
      • 1 = 1
      • 30.15 = 30
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Part 3
Part 3 of 3:

Understanding Chemical Formulae

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  1. An empirical formula gives you information about the molar ratios of one atom to another in a molecule. This does not provide any information about exactly how many atoms are present in the molecule. The empirical formula also fails to provide information about the structure and bonding of atoms in a molecule. [8]
  2. Like the empirical formula, the molecular formula fails to provide information about the bonding and structure of a molecule. Unlike the empirical formula, the molecular formula gives you details about how many of each atom is present in the molecule. The empirical formula and molecular formula are related by a whole number ratio. [9]
  3. Structural representations give even more information than molecular formulas. In addition to showing how many of each atom is present in a molecule, structural representations give you information about the bonding and structure of the molecule. This information is crucial to understanding the way the molecule will react. [10]
    • There are several different types of structural representations, which show you different things about the compound. For example, it might show the compound's connectivity or its molecular shape, such as by drawing dashed lines to indicate their bonds.
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  • Question
    How do I find molecular formulas?
    Meredith Juncker, PhD
    Scientific Researcher
    Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.
    Meredith Juncker, PhD
    Scientific Researcher
    Expert Answer
    You can follow the steps above. Start by finding the empirical formula. You should then calculate the empirical formula weight. Then, divide the molecular formula weight by the empirical formula weight. This answer gives you the number you'll need to multiply the subscripts in the empirical formula to get the molecular formula. You can double check that you got the right answer by calculating the molecular molar mass of the molecular formula you calculated.
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  • Question
    A compound contains 3.2g of Cu, 0.6g of C and 2.4 O2. How do I find its empirical formula?
    Community Answer
    You would first need to convert each of the gram measurements to moles. Accomplish this by dividing each by its respective atomic mass on the periodic table. Then divide each measurement by the lowest mole amount. You can continue doing this until you get whole number subscripts.
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    If the molar mass is 99 g/mol, what is the molecular formula?
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    We do not have enough information. It could be multiple elements in different ratios.
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      • Keep in mind that the lowest number of atoms in a molecule for some elements might not be 1. For example, a ratio could be 1.333, which as a fraction is 4/3. To get rid of your fraction, you can multiply by the denominator, in this case 3.
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      Things You'll Need

      • Periodic table of elements
      • Calculator
      • Pencil
      • Paper

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      References

      1. ↑ https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties/x2eef969c74e0d802:elemental-composition-of-pure-substances/v/empirical-molecular-and-structural-formulas
      2. ↑ https://www.khanacademy.org/science/physics/thermodynamics/temp-kinetic-theory-ideal-gas-law/a/what-is-the-ideal-gas-law
      3. ↑ https://science.widener.edu/~svanbram/ptable_6.pdf
      4. ↑ https://antoine.frostburg.edu/chem/senese/101/gases/faq/molecular-formula-cyclopropane.shtml
      5. ↑ https://antoine.frostburg.edu/chem/senese/101/gases/faq/molecular-formula-cyclopropane.shtml
      6. ↑ https://antoine.frostburg.edu/chem/senese/101/gases/faq/molecular-formula-cyclopropane.shtml
      7. ↑ https://www.youtube.com/watch?v=FZhWVy8INyk
      8. ↑ https://www.khanacademy.org/math/ap-statistics/density-curves-normal-distribution-ap/stats-normal-distributions/e/empirical_rule
      9. ↑ https://www.youtube.com/watch?v=wnRaBWvhYKY
      1. ↑ https://www.khanacademy.org/science/biology/properties-of-carbon/carbon/v/representing-structures-of-organic-molecules

      About This Article

      Article Summary X

      To find a molecular formula, start by calculating the number of moles and the molecular weight of the gas using their respective formulas. Then, add together the atomic weight of each of the atoms in the formula, which you can find using the periodic table. Next, divide the molecular weight by the empirical formula weights to come up with a ratio. Finally, multiply the entire empirical formula by this ratio to yield the molecular formula. For tips from our Science reviewer on how to remember the differences between a molecular formula and an empirical formula, read on!

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