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, which can be determined from the compound's molar mass. For example, glucose has an empirical formula of CH₂O and an n factor of 6, resulting in a molecular formula of C₆H₁₂O₆. Understanding these concepts is crucial for accurate chemical analysis and stoichiometry.

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 C1H2O1. We have C4H9 and C7H6O3. In order to determine their molecular formulas, we can use n factors. N factors are just numbers that we can multiply by the empirical formula 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 will give me my molecular formula. So that'd be 6 carbons, 12 hydrogens, and 6 oxygens. So the molecular formula of glucose is C6H12O6. Octane 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 different n factors for any given compound once you've determined its empirical formula.

Molecular Formula Example 1

The key to defining your molecular formula is first determining 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 carbon, 6.7 grams of hydrogen, 53.3 grams of oxygen. Next, we're going to convert each one of those grams into moles. So 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 carbon, 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 hydrogen and 3.3313 moles of oxygen. 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 going to give me one carbon, two hydrogens, one oxygen. So my molecular 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 it's made up of 1 carbon, 2 hydrogens, 1 oxygen. Multiply each one by their atomic masses from the periodic table, and then we're going to add up those totals together. So when we add them 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 going to multiply the subscripts of the empirical formula by the n-factor to get the molecular formula. So here we're going to say that the n-factor times our empirical formula equals our molecular formula. Right? So then we're going to plug in the 3 that we just found times the empirical formula which you discovered earlier which 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 an empirical formula and a molecular formula?

The empirical formula represents the simplest whole-number ratio of elements in a compound, while the molecular formula shows the actual number of each type of atom in a molecule. For example, the empirical formula of glucose is CH₂O, indicating a 1:2:1 ratio of carbon, hydrogen, and oxygen. However, the molecular formula of glucose is C₆H₁₂O₆, showing that each molecule contains 6 carbon, 12 hydrogen, and 6 oxygen atoms. Understanding both formulas is crucial for accurate chemical analysis and stoichiometry.

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

To determine the molecular formula from the empirical formula, you need the molar mass of the compound. First, calculate the molar mass of the empirical formula. Then, divide the compound's molar mass by the empirical formula's molar mass to find the n factor. Multiply each subscript in the empirical formula by this 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?

An n factor is a multiplier used to convert an empirical formula to a molecular formula. It is determined by dividing the molar mass of the compound by the molar mass of the empirical formula. For example, if the empirical formula is CH₂O and the molar mass of the compound is 180 g/mol, while the molar mass of CH₂O is 30 g/mol, the n factor is 6. Thus, the molecular formula is C₆H₁₂O₆.

Why is it important to know the molecular formula of a compound?

Knowing the molecular formula of a compound is important because it provides the exact number of each type of atom in a molecule, which is essential for understanding its chemical properties and behavior. This information is crucial for tasks such as predicting reactions, calculating stoichiometry, and designing chemical syntheses. For example, the molecular formula C₆H₁₂O₆ tells us that glucose has 6 carbon, 12 hydrogen, and 6 oxygen atoms, which is vital for biochemical studies and applications.

Can the empirical formula and molecular formula be the same?

Yes, the empirical formula and molecular formula can be the same if the compound's simplest ratio of elements is also its actual number of atoms. This occurs when the n factor is 1. For example, the empirical formula of water (H₂O) is the same as its molecular formula because the ratio of hydrogen to oxygen is 2:1, and each molecule of water contains exactly 2 hydrogen atoms and 1 oxygen atom.