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  • The Molecular Formula tells you the number of atoms of each of the elements in one molecule

  • of a compound.

  • In a previous video, we found theempirical formulaand this is a related idea, but

  • they are not the same thing.

  • The Empirical Formula gives the lowest whole number ratio of the atoms of the elements

  • in a compound.

  • This may or may not be the same as the molecular formula for a given compound.

  • The molecular formula is sometimes called thetrue formulabecause it tells you

  • how many of each kind of atom are actually present in the molecule.

  • It’s important to note that in a molecule, the arrangements of the atoms in 3 dimensions

  • makes a big difference.

  • Neither the molecular formula nor the empirical formula spell out what the molecule looks

  • like in three dimensions.

  • For that, we need a structural formula, and

  • well save that complicated idea for another video

  • So what IS the relationship between these two kinds of chemical formulas?

  • The molecular formula is either the same as the empirical formula or it is a simple whole-number

  • multiple of the empirical formula.

  • It’s going to have the same ratio of atoms as the empirical formula, but the actual number

  • of atoms may be different.

  • For example, formaldehyde and acetic acid have the same empirical formula, CH2O, but

  • different molecular formulas.

  • Formaldehyde is an interesting case, because its molecular formula is exactly the same

  • as its empirical formula.

  • This tells you the actual number of each kind of atom in each molecule of formaldehyde :

  • 1 atom of carbon, 2 atoms of hydrogen, and 1 atom of oxygen.

  • By contrast, the molecular formula of acetic acid is a multiple of its empirical formula.

  • If you multiply the empirical formula by 2, you get the molecular formula of acetic acid:

  • C2H4O2.

  • Notice the ratio of atoms is the same.

  • But in each molecule of acetic acid, there are actually

  • 2 atoms of Carbon, 4 atoms of Hydrogen, and 2 atoms of Oxygen.

  • To find the molecular formula of a compound, you need to have its empirical formula and

  • its molar mass.

  • The goal is to find out how many empirical formula units are in one molecule of the compound.

  • Our strategy is to Calculate the empirical formula mass

  • Divide the molar mass by the empirical formula mass.

  • This tells you how many of the empirical formula units are needed to form

  • one molecule of the compound.

  • This should be a whole number answer, or very close.

  • If your answer isn’t an integer or very close to one, youve made a mistake somewhere.

  • Multiply the empirical formula by this value.

  • Let’s see some examples:

  • Example 1: A compound has the empirical formula CH2O.

  • It has a molar mass of 180 g/mol.

  • What is the molecular formula?

  • This empirical formula looks familiar - it’s the same empirical formula as formaldehyde

  • and acetic acid, as we talked about earlier.

  • Let’s go through the steps and see if it’s one of those.

  • STEP 1: Calculate the empirical formula mass.

  • We get that from the periodic table.

  • 12.011 + 2(1.008) + 15.999 = 30.026 g/mol

  • STEP 2: Divide the molar mass by the empirical formula mass.

  • 180 g/mol / 30.026 g/mol = 5.99 Were looking for a simple whole number to multiply the

  • empirical formula by, so well round to 6.

  • STEP 3: Multiply the empirical formula by this value: 6 times (CH2O) = C6H12O6.

  • This is glucose.

  • So now we know 3 compounds that have the same empirical formula,

  • but different molecularformulas.

  • Example 2: For our next example, let’s work a problem all the way through from the percent

  • composition data and the molar mass to get the molecular formula.

  • This means first we will find the empirical formula, and then go on to find the molecular

  • formula.

  • Our experimental data tells us the compound is 94.1% O and 5.9% H. The molar mass is 34g/mol.

  • First, well find the empirical formula, which only requires the % composition data.

  • Assume 100g.

  • Then we have 94.1g Oxygen and 5.9g Hydrogen.

  • Convert to moles to get the mole ratio.

  • For oxygen, we have 94.1g (1 mol/15.999g) = 5.88 mol oxygen

  • For hydrogen, we have 5.9g (1 mol/1.008g)= 5.85 mol Hydrogen

  • H 5.85 O 5.88 Divide through by the lowest number, 5.85.

  • We get H1O1 (remember the 1s are understood, so we

  • get HO as the empirical formula.

  • Next we find the empirical formula mass.

  • 1.008 g/mol + 15.999 g/mol = 17.007 g/mol

  • Now we use the molar mass given in the problem.

  • Molar mass/ empirical formula mass = 34.0 g/mol / 17.007 g/mol = 2

  • In other words, there are 2 empirical formula units per 1 of these molecules.

  • So we multiply the empirical formula by 2: 2(HO) = H2O2 hydrogen peroxide.

  • What if you see a problem asking you to go in the other direction?

  • Given a molecular formula, what is the empirical formula?

  • Example 3: Butane has the molecular formula C4H10.

  • What is the empirical formula?

  • Notice there is no other information given in the problem.

  • We don’t need any percent composition data or molar mass information to solve this problem.

  • The empirical formula is going to be the molecular formula divided by the Greatest Common Divisor

  • of the subscripts.

  • The subscripts here are 4 and 10.

  • If you factor these, 4 is 2x2, and 10 is 2x 5.

  • The greatest common divisor is 2, so we will divide through by 2 to simplify the molecular

  • formula down to the empirical formula.

  • C4/2 H10/2 = C2H5.

  • Remember, an empirical formula may or may not be the same as the molecular formula.

  • The molecular formula is always a simple multiple of the empirical formula.

  • For example, for the empirical formula CH2O, there are many possible multiples that correspond

  • to different compounds.

  • CH2O is both the empirical formula and molecular formula for formaldehyde, but if we multiply

  • that empirical formula by 2, we get C2H4O2,

  • which is the molecular formula for acetic acid.

  • If we multiply CH2O by 6, we get C6H12O6, which is glucose.

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The Molecular Formula tells you the number of atoms of each of the elements in one molecule

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    林宜悉 發佈於 2021 年 01 月 14 日
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