Molecular Formula - Online Tutor, Practice Problems & Exam Prep

The molecular formula represents the actual number of each element in a compound, while the empirical formula shows the relative number of elements. To find the molecular formula, multiply the empirical formula by an n factor. For example, glucose (C₆H₁₂O₆) has an n factor of 6, octane (C₈H₁₈) has an n factor of 2, and salicylic acid (C₇H₆O₃) has an n factor of 1. Determining the n factor involves specific steps that will be explored further.

Molecular Formula

Recall that the molecular formula gives the actual number of different elements in a given compound, meaning it gives us the real formula for the compound. The empirical formula, on the other hand, just gives us the relative number of those different elements in that same compound. Up to this point, we've learned to calculate the empirical formula when given either the masses or the percentages of those elements within that compound. But how exactly would we determine the molecular formula for those same compounds? Well, here we have a chart. And in this chart, we're talking about 3 compounds: glucose, octane, and salicylic acid. We're going to say that their empirical formulas are given as CH₂O, C₄H₉, and C₇H₆o₃. In order to determine their molecular formulas, we can use n factors. N factors are just numbers that we can multiply the empirical formula by in order to determine the molecular formula. So for example, glucose has an n factor of 6. That means I will multiply each of the subscripts of the empirical formula by 6, and that'll give me my molecular formula. So that'd be 6 carbons, 12 hydrogens, and 6 oxygens. The molecular formula of glucose is C₆H₁₂O₆. Octane, it has an n factor of 2. So multiplying those subscripts gives me C₈H₁₈. And then finally, salicylic acid has an n factor of 1, which means that the molecular formula and empirical formula are the same. So just remember, when you're given the n factor, just multiply the empirical formula by it, and you'll determine what your molecular formula will be. But what happens when we don't have the n factor? How do we determine it? In the next series of videos, we'll go through the different steps required to find the n factors for any given compound once you've determined its empirical formula.

Molecular Formula Example 1

The key to determining your molecular formula is first finding your empirical formula. Now, we're going to say, once the empirical formula is determined, the molecular formula can be obtained if the molar mass is also known. Realize here that the molar mass is connected to the molecular formula. If we take a look here at this example question, it says, after a workout session, lactic acid, which has a molar mass of 90.08 grams per mole, forms in muscle tissue and is responsible for muscle soreness. Elemental analysis shows that the compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen. Determine the molecular formula. Alright. To determine the molecular formula, we follow step 1. In step 1, it says repeat the steps necessary to determine the empirical formula of the compound. Remember, what we do first is we're going to convert all of these percentages immediately into grams, because we assume that we're dealing with 100 grams of our compound. So that's going to be 40 grams of C, 6.7 grams of H, 53.3 grams of oxygen. Next, we're going to convert each one of those grams into moles. One mole of each of these elements can be determined by finding their atomic masses on the periodic table. 1 mole of carbon weighs 12.01 grams according to the periodic table. Hydrogen is 1.008 grams. Oxygen is 16 grams. Here, all the grams cancel out and we'll have moles of each of the elements. So here we have 3.3306 moles of C, and remember, when it comes to finding the moles at this point, to avoid rounding errors, make sure you have at least 4 decimal places. Next, we have 6.6468 moles of H, and this is 3.3313 moles of O. Now remember at this point, we're going to divide all the moles by the smallest mole answer in order to determine the ratios to one another. The smallest mole answer that we got was 3.3306. So all of them will be divided by this number. So that's gonna give me one carbon, two hydrogens, one oxygen. So my empirical formula is CH₂O. With the empirical formula determined, we're now going to do step 2. Step 2 says we calculate the empirical mass of the compound. The empirical mass comes from the empirical formula which we just discovered. So that would be it's made up of 1 carbon, 2 hydrogen, 1 oxygen. Multiply each one by their atomic masses from the periodic table, and then we're gonna add up those totals together. So when we add those all together, we're going to say the empirical mass of our compound is 30.026 grams per mole. This takes us to step 3. Step 3 says we divide the molar mass, which was given to us within the example problem, of the molecular formula by the empirical mass to determine the n factor. So our n factor equals our molar mass, which was given to us within the question, divided by the empirical mass, which we can only determine once we find the empirical formula. So that would be 90.08 grams per mole divided by 30.026 grams per mole, which gives us an n factor equal to 3. Now we're gonna multiply the subscripts of the empirical formula by the n factor to get the molecular formula. So here we're going to say that n factor times our empirical formula equals our molecular formula. Right? So, then we're gonna plug in the 3 that we just found times the empirical formula which you discovered earlier equals C3H6O3. This gives us the molecular formula for this question. So just remember, the key to determining the molecular formula is to first know what your empirical formula is. Once you have that, determine your n factor. And then n factor times empirical formula gives us our molecular formula.

What is the molecular formula for the following compound? 

Empirical Formula: NPCl₂                     Molar Mass:347.64 g/mol

• A. NPCl₂
• B. N₃P₃Cl₆
• C. N₂P₂Cl₂
• D. N₄P₄Cl₈

Cortisol (MW = 362.47 g/mol), a known steroid hormone, is found to contain 69.6% carbon, 8.34% hydrogen, and 22.1% oxygen by mass. What is its molecular formula?

• A. C₂₁H₃₀O₅
• B. C₂₁H₂₆O₅
• C. C₂₈H₄₆O
• D. C₁₈H₂₄O₂

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

Here’s what students ask on this topic:

What is the difference between a molecular formula and an empirical formula?

The molecular formula represents the actual number of each type of atom in a molecule of a compound. For example, the molecular formula of glucose is C₆H₁₂O₆. The empirical formula, on the other hand, shows the simplest whole-number ratio of the elements in the compound. For glucose, the empirical formula is CH₂O. While the molecular formula provides detailed information about the molecule's composition, the empirical formula gives a simplified version that reflects the relative proportions of the elements.

How do you determine the molecular formula from the empirical formula?

To determine the molecular formula from the empirical formula, you need to know the n factor, which is the ratio of the molecular formula mass to the empirical formula mass. First, calculate the empirical formula mass by summing the atomic masses of all atoms in the empirical formula. Then, divide the molecular formula mass (usually given) by the empirical formula mass to find the n factor. Finally, multiply each subscript in the empirical formula by the n factor to get the molecular formula. For example, if the empirical formula is CH₂O and the n factor is 6, the molecular formula is C₆H₁₂O₆.

What is an n factor in the context of molecular formulas?

The n factor is a multiplier used to convert an empirical formula to a molecular formula. It is determined by dividing the molecular formula mass by the empirical formula mass. For example, if the empirical formula of a compound is CH₂O and its molecular formula mass is 180 g/mol, while the empirical formula mass is 30 g/mol, the n factor is 180/30 = 6. This means the molecular formula is obtained by multiplying each subscript in the empirical formula by 6, resulting in C₆H₁₂O₆.

How do you find the empirical formula from the masses or percentages of elements in a compound?

To find the empirical formula from the masses or percentages of elements, follow these steps: 1) Convert the mass or percentage of each element to moles by dividing by the atomic mass of the element. 2) Determine the simplest whole-number ratio of the moles of each element. 3) If necessary, multiply the ratios by a common factor to get whole numbers. For example, if a compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen, convert these to moles (C: 40/12, H: 6.7/1, O: 53.3/16), find the simplest ratio, and adjust to whole numbers to get the empirical formula CH₂O.

Why is the molecular formula important in chemistry?

The molecular formula is important in chemistry because it provides the exact number of each type of atom in a molecule, which is crucial for understanding the compound's properties, behavior, and reactions. Unlike the empirical formula, which only gives the simplest ratio of elements, the molecular formula allows chemists to determine the molecular structure, predict reactivity, and calculate the molecular weight. For example, knowing that glucose has the molecular formula C₆H₁₂O₆ helps in understanding its role in biological processes and its energy content.