Molecular Formula: Videos & Practice Problems

Understanding the relationship between empirical and molecular formulas is crucial in chemistry. The empirical formula represents the simplest whole-number ratio of elements in a compound, while the molecular formula indicates the actual numbers of atoms present. To convert an empirical formula to a molecular formula, one can use the "n factor," which is multiplied by the subscripts in the empirical formula. For instance, glucose with an empirical formula of CH₂O and an n factor of 6 yields the molecular formula:

$C_6{H}_{12}{O}_6$

If the n factor is unknown, additional steps are required to calculate it, which involves determining the compound's empirical formula and molar mass. This process allows chemists to accurately identify the composition of substances and understand their properties.

Determining Molecular Formulas

The molecular formula of a compound provides the actual number of each type of atom present, while the empirical formula indicates the relative number of those atoms. To derive the molecular formula from the empirical formula, we utilize a concept known as the n factor, which is a multiplier that adjusts the subscripts in the empirical formula to yield the molecular formula.

For instance, consider three compounds: glucose, octane, and salicylic acid. Their empirical formulas are as follows:

• Glucose: CH₂O
• Octane: C₄H₉
• Salicylic acid: C₇H₆O₃

To find the molecular formulas, we first determine the n factor for each compound:

• For glucose, the n factor is 6. Thus, multiplying the subscripts of the empirical formula by 6 results in C₆H₁₂O₆.
• For octane, the n factor is 2, leading to the molecular formula C₈H₁₈.
• For salicylic acid, the n factor is 1, meaning the molecular formula is the same as the empirical formula: C₇H₆O₃.

In summary, to determine the molecular formula, simply multiply the subscripts of the empirical formula by the n factor. If the n factor is not provided, the next steps involve calculating it based on the compound's molar mass and the empirical formula's molar mass, which will be explored in subsequent discussions.

Molecular Formula Calculation Example

To determine the molecular formula of a compound, one must first establish its empirical formula and know its molar mass. For instance, consider Ibuprofen, which has a molar mass of 206.3 grams per mole and a percent composition of 75.70% Carbon (C), 8.80% Hydrogen (H), and 15.50% Oxygen (O). The process begins by converting these percentages into grams, assuming a 100-gram sample. This results in 75.70 grams of Carbon, 8.80 grams of Hydrogen, and 15.50 grams of Oxygen.

Next, to find the number of moles of each element, divide the mass of each by its atomic mass, which can be found on the periodic table. For Carbon, the atomic mass is approximately 12 grams per mole, for Hydrogen it is about 1 gram per mole, and for Oxygen, it is 16 grams per mole. This calculation yields approximately 6.3031 moles of Carbon, 8.7302 moles of Hydrogen, and 0.9688 moles of Oxygen.

To simplify these mole values into a ratio, divide each by the smallest number of moles, which in this case is 0.9688. This results in a ratio of approximately 6.5 for Carbon, 9 for Hydrogen, and 1 for Oxygen. Since the ratio must be whole numbers, multiply all ratios by 2 to eliminate the decimal, resulting in 13 Carbons, 18 Hydrogens, and 2 Oxygens. Thus, the empirical formula is C₁₃H₁₈O₂.

The next step is to calculate the empirical mass of the compound using the empirical formula. This involves multiplying the number of each atom by its atomic mass: 13 Carbons (156.13 grams), 18 Hydrogens (18.144 grams), and 2 Oxygens (32 grams). Adding these values gives an empirical mass of approximately 206.274 grams per mole.

To find the n factor, divide the known molar mass of Ibuprofen (206.3 grams per mole) by the empirical mass (206.274 grams per mole). This calculation yields an n factor of approximately 1. Since n equals 1, the molecular formula is the same as the empirical formula, which is C₁₃H₁₈O₂.

In summary, the molecular formula can be derived from the empirical formula by determining the n factor based on the molar mass. In this case, both the empirical and molecular formulas for Ibuprofen are identical, demonstrating that sometimes they can be the same when n equals 1.

Use the given empirical formula and molar mass to determine the molecular formula.
Empirical Formula: NH₂                      Molar Mass: 32.052 g/mol

• A. NH₄
• B. N₄H
• C. NH₂
• D. N₂H₄

Answer: (D)

Glyceraldehyde (M = 90.078 g/mol), a simple monosaccharide, is comprised of 39.999% C, 6.714% H, and 53.297% oxygen by atomic weight. What would be its molecular formula?

• A. C₃H₆O₃
• B. CH₂O
• C. C₃HO₃
• D. C₂H₈O₂

Answer: (A)

Elemental analysis of a pure compound indicated that the compound had 72.2% C, 8.50% H and the remainder as O. If 0.250 moles of the compound weighs 41.55 g, what is the molecular formula of the compound?

• A. C₅H₇O
• B. C₁₀H₁₄O₂
• C. C₆H₈O₂
• D. C₁₂H₁₆O₄

Answer: (B)

Here’s what students ask on this topic:

How can I find the molecular formula?

To find the molecular formula, you first need to determine the empirical formula, which represents the simplest whole-number ratio of elements in the compound. Then, you'll need the molecular weight (or molar mass) of the compound, which can be found using a mass spectrometer or calculated from the atomic weights of its constituent elements.

Here's a step-by-step guide:

1. Obtain the Percent Composition: If you're given percentages of each element in the compound, assume a 100-gram sample so that the percentages can directly translate into grams.
2. Convert to Moles: Use the atomic masses of the elements (from the periodic table) to convert the mass of each element into moles.
3. Find the Simplest Ratio: Divide the moles of each element by the smallest number of moles to find the simplest whole-number ratio.
4. Determine the Empirical Formula: Use the ratios to write the empirical formula.
5. Calculate the Empirical Formula Mass: Add up the atomic masses of all the atoms in the empirical formula.
6. Find the Ratio of Molecular Mass to Empirical Formula Mass: Divide the molecular weight of the compound by the empirical formula mass.
7. Determine the Molecular Formula: Multiply the subscripts in the empirical formula by the ratio calculated in the previous step to get the molecular formula.

How can the molecular formula be found from the empirical formula?

To find the molecular formula from the empirical formula, you need to determine the molecular mass of the compound and compare it with the empirical formula mass. Here's how you do it:

1. Calculate the empirical formula mass by adding up the atomic masses of all the atoms in the empirical formula.
2. Obtain the molecular mass of the compound. This is often done using experimental data from techniques like mass spectrometry.
3. Divide the molecular mass by the empirical formula mass to find the ratio between them. This ratio should be a whole number or very close to a whole number.
4. Multiply all the subscripts in the empirical formula by this ratio to get the molecular formula.

For example, if the empirical formula is CH₂ and the molecular mass is found to be approximately 56 g/mol, the empirical formula mass (12 for C + 2*(1 for H)) is 14 g/mol. Dividing the molecular mass by the empirical formula mass gives 4. Therefore, the molecular formula is C₄H₈, which is four times the empirical formula.

What is the molecular formula?

A molecular formula represents the actual number of atoms of each element in a molecule of a compound. It is a way of expressing the chemical composition of a molecular substance using the symbols for the elements and numerical subscripts to indicate the number of atoms of each element present in the molecule. For instance, the molecular formula for water is:

$H_{2}O$

which tells us that each molecule of water is composed of two hydrogen atoms and one oxygen atom. In contrast to empirical formulas, which show the simplest whole-number ratio of atoms in a compound, the molecular formula provides specific information about the molecular structure. This can be particularly important for distinguishing between isomers—compounds that have the same empirical formula but different molecular structures and properties.

How do you calculate the molecular formula?

To calculate the molecular formula, you first need to determine the empirical formula, which represents the simplest whole-number ratio of elements in a compound. Here's a step-by-step process:

1. Obtain the percent composition of each element in the compound, either from experimental data or given information.
2. Convert these percentages to grams (assuming a 100-gram sample) to make calculations easier.
3. Convert the grams of each element to moles by dividing by the atomic mass of each element (from the periodic table).
4. Find the simplest whole-number ratio of moles of each element by dividing all the mole amounts by the smallest number of moles calculated.
5. This ratio gives you the empirical formula.

To find the molecular formula, you need the molecular weight (or molar mass) of the compound, which can be determined experimentally or provided.

6. Divide the molecular weight by the empirical formula weight (the sum of all the atomic masses in the empirical formula).
7. This gives you a whole number, which you multiply by the subscripts in the empirical formula to get the molecular formula.

For example, if the empirical formula is CH₂ and the molecular weight is 56 g/mol, and the empirical formula weight is 14 g/mol, then the molecular formula is C₄H₈ (56/14 = 4, so multiply each subscript by 4).