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2.11: Empirical and Molecular Formulas

  • Page ID
    158413
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    learning objectives 

    1. Distinguish between empirical formula and molecular formula 

    2. Determine empirical formula and molecular formula using percent composition 

    3. Determine empirical formula and molecular formula using mass data

     

    Empirical and Molecular Formulas

    Empirical measurements are based on a measurable quantity like mass. Knowing the mass of each element in a compound we can determine it's formula. There are two types of formulas, empirical and molecular.

    Empirical Formula: Lowest whole number ratio of the elements in a compound

    Molecular Formula: Actual whole number ratio of the elements in a compound.

    For salts that do not have homonuclear diatomic ions (like Hg2+2 or O2-2) the empirical formula is the formula we write to describe the salt. But multiple molecules can have the same empirical formula. For example, benzene (C6H6) and acetylene (C2H2) both of the empirical formula of CH.

    Empirical Formulas:

    Steps:

    1. Obtain Mass of Each Element (in grams)

    - if given % composition assume 100 g and convert to mass.

    - if given total mass you need the mass of all but one element (sum of masses equals total mass)

    - if given % composition you need that of all but one element (sum of percents equals 100 %)

    2. Calculate # of moles of each element present from masses and atomic weights

    - You now have a formula representing the mole ratio of the elements in the compound and you need to make these integers.

    3. Divide # of Moles of each element by the one with the smallest value (forcing it to one and making all other values greater than one).

    4. Multiply the results of step 3 by the smallest integer which will convert them all to whole numbers.

    Trick: convert decimals to fractions and multiply by lowest common denominator. 

     

    Video Tutor: 

     

    What is the Empirical Formula for Aspirin if it is 60.0% C and 35.5% O?

    Aspirin

    Video 2.14a: Empirical formula of aspirin

    Example:

    A certain compound was found to contain 67.6% C, 22.5% O, and 9.9% H. What is the empirical formula?

    \[22.5gO\left ( \frac{1molO}{16.00g} \right )= 1.4\Rightarrow \frac{1.4}{1.4}= 1\]

    \[67.6gC\left ( \frac{1molC}{12.011g} \right )= 5.63\Rightarrow \frac{5.63}{1.4}= 4\]

    \[9.9gH\left ( \frac{1molH}{1.007g} \right )= 9.9\Rightarrow \frac{9.9}{1.4}= 7\]

    C4H7O

    This Applet comes from the ChemCollective at Carnegie Mellon University. This link will send you to the video and tutorial associated with this applet. Every time you load the page a new problem will load, and there are a series of tiered hints to help you work through the problems.

    Molecular Formulas:

    The empirical formula represents the lowest whole number ratio of the elements in a molecule while the molecular formula represents the actual formula of the molecule.Both Benzene (C6H6, molar mass = 78.12g/mol) and acetylene (C2H2, molar mass = 26.04g/mol) have the same percent composition (92.24 mass% carbon and 7.76% hydrogen) and the empirical formula, CH. The ratio of the atoms in the actual molecule is an integer multiple of those in the empirical formula (acetylene has twice the number of atoms and benzene has 6 times the number in the empirical formula). Likewise, the molar mass of a molecule will be an integer times that of the formula mass of the empirical formula (n = 1,2,3,4..?, note if these equal, n=1).

    Trick: Calculate the formula mass of the empirical formula and divide this into the molar mass of the compound. This will give you the integer which when multiplied by the empirical formula gives you the molar formula.

     

    Example:

    A certain compound was found to contain 67.6% C, 22.5% O, and 9.9% H. If the molecular weight of the compound was found to be approximately 142 g/mol, what is the correct molecular formula for the compound?

    C4H7O has a empirical weight of

    (4)(12.01) + (7)(1.007) + (16.00) = 71.09g/mol

    \[\left ( \frac{142g/mol}{71.09g/mol} \right )= 2\]

    multiply the coefficients by 2

    C8H14O2

    Practice

     

    1. You have a 3.3700 g sample of a salt which contains copper, nitrogen and oxygen. It contains 1.1418 g of copper and 1.7248 g of oxygen. What is the empirical formula of the salt?

    2. What is the empirical formula of a compound that contains 30.43% N and 69.57% O by weight?

    3. What is the empirical formula of a compound that contains 87.5% N and 12.5% H by weight?

    4. An unknown compound is made up of C, H, N and O. A 2.402-g sample of this compound contains 1.121 g of N, 0.161 g H, 0.480 g C, and an unspecified amount of oxygen. What is the empirical formula of the unknown compound?

    5. A substance was determined to be 49.9% oxygen, 37.5% carbon and 12.6% hydrogen by mass. What is the proper empirical formula for this compound?

    6. A compound has an empirical formula of C2HF has a molar mass of 132.06 g/mol. What is the molecular formula for the compound?

    7. What is the molecular formula if a 200.0 g sample of an acid with a molar mass of 616.73g/mol contains 171.36 g of carbon, 18.18g of nitrogen and the rest is hydrogen?

    8. Strychine has a molar mass of 334 g/mol and percent composition of 75.42%C, 6.63%H and 8.38%N and the rest oxygen. What is the molecular formula of strychine?

    Answers

    1. CuN2O6

    2. NO2

    3. NH2

    4. N2H4CO

    5. CH4O

    6. C6H3F3

    7. C44N4H32

    8. C21N2H22O2

    Contributors

    • Bob Belford (UALR) and November Palmer (UALR)
    • Modified by Ronia Kattoum (UA of Little Rock) 

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     


    2.11: Empirical and Molecular Formulas is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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