How to Calculate Molecular Weight

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The molecular mass, often called the molecular weight (MW), is the weight of all atoms in a given molecular formula . Molecular weight is measured in Atomic Mass Units, usually expressed as u or amu. [1] In order to calculate the molecular weight of a formula, you'll need to add up the atomic masses of each element present.

Method 1
Method 1 of 2:

Calculating Molecular Weight

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  1. First, list each element present in the molecule. You may use the chemical symbol or write out the name of the element. Then, count the atoms according to the molecular formula and write them next to the element’s name or symbol. [2]
    • For example, carbon dioxide or CO 2 , list that there is 1 carbon (C) and 2 oxygens (O) present in the molecule.
    • For methane, CH 4 , list carbon (C) and hydrogen (H). Methane comprises 1 atom of carbon and 4 atoms of hydrogen.
    • For glucose, C 6 H 12 O 6 , list carbon (C), hydrogen (H), and oxygen (O). Glucose comprises 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
  2. Find the relative atomic mass of each element in the molecule. Use a copy of the Periodic Table of Elements . The Periodic Table lists the atomic mass of each element below the chemical symbol. For example, oxygen has a relative atomic mass of 15.9994 amu. [3]
    • For carbon dioxide (CO 2 ), the relative atomic mass is 12.011 amu for carbon and 15.999 for oxygen.
    • The atomic mass of an element is roughly equal to the summed mass of the protons and neutrons that it contains. Note that the relative atomic mass you find in the periodic table is scaled: it accounts for all of the isotopes of the element in the proportions that they naturally occur. [4]
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  3. Multiply the atomic mass of each element by the number of atoms of that element: (Atomic Mass of Element) x (# of atoms of that element). Do this for each element in the molecule. [5]
    • In our carbon dioxide example, the mass of the single carbon atom is 12.011 amu. Since there are 2 oxygen atoms, you would write .
  4. Molecular weight = ∑ ( (Atomic Mass of Element) n x (# of atoms of that element) n ) [6]
    • Round the answer as necessary, using significant digits.
    • Remember to use the proper units. amu is the old abbreviation for atomic mass units, but the "most correct" modern unit is a lower-case u . [7]
    • For carbon dioxide, the molecular weight can be found like so: .
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Method 2
Method 2 of 2:

Practicing with Examples

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  1. The atomic mass of hydrogen is 1.008 u. The atomic mass of carbon is 12.011 u. [8] There are 4 hydrogen atoms in methane. Add together the weight of all the atoms in methane to find the molecular weight. [9]
    • .
  2. The atomic mass of carbon is 12.011 u. The atomic mass of hydrogen is 1.008 u. The atomic mass of oxygen is 15.999 [10] There are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Add up the mass of all of the atoms in glucose to calculate the molecular weight.
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  • Question
    What is the molecular mass of phosphorus?
    Community Answer
    The molecular mass of Phosphorus is 30.974 to 3 decimal places, commonly written as 31 on the periodic table.
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  • Question
    Why are the hydrogen bonds weak?
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    Hydrogen has a low electronegativity, and thus is easily taken out of bonds.
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  • Question
    What is the molecular weight of CO2?
    Williamsm917
    Community Answer
    44.01 g/mol. C has a molecular mass of 12.01. O has a molecular mass of 16. Therefore, the formula would be 12.01 + 2(16), which gives you the molecular mass 44.01 g/mol.
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      References

      1. ↑ http://www.chemteam.info/Mole/MolecWt.html
      2. ↑ https://www.cmu.edu/gelfand/lgc-educational-media/polymers/what-is-polymer/molecular-weight-calculation.html
      3. ↑ http://www.ptable.com/
      4. ↑ https://www.khanacademy.org/science/biology/chemistry--of-life/elements-and-atoms/a/atomic-number-atomic-mass-and-isotopes-article
      5. ↑ https://chem.libretexts.org/Courses/University_of_British_Columbia/CHEM_100%3A_Foundations_of_Chemistry/06%3A_Chemical_Composition/6.9%3A_Calculating_Molecular_Formulas_for_Compounds
      6. ↑ http://chemcollective.org/activities/tutorials/stoich/calculating_molecular_weight
      7. ↑ http://www.chemteam.info/Mole/MolecWt.html
      8. ↑ http://www.ptable.com/
      9. ↑ https://www.khanacademy.org/science/chemistry/atomic-structure-and-properties/introduction-to-compounds/v/molecular-mass-and-molecular-weight
      1. ↑ http://www.ptable.com/

      About This Article

      Article Summary X

      To calculate molecular weight, start by listing all elements present in the molecule. Then, count the atoms using the molecular formula. For example, use the formula CH4 for methane, which comprises 1 atom of carbon and 4 atoms of hydrogen. In a copy of the Periodic Table of Elements, find the relative atomic mass of each element in the molecule. Then, multiply the atomic mass of an element by the number of atoms of the element in the molecule. To complete the calculation, add up the mass of all of the atoms to get the molecular weight. If you want to learn the units to use when you label your calculations, keep reading the article!

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