Calculating Molar Mass: Study with Video Lessons, Practice Problems & Examples

Molar mass is a physical property defined as the mass of a substance divided by the amount of that substance, typically expressed in grams per mole (g/mol). It is essential to understand that molar mass, also known as molar weight, molecular weight, or molecular mass, represents the relationship between mass and the amount of substance. This concept is crucial in various chemical calculations, including stoichiometry and determining the yield of reactions. Remembering the formula for molar mass is vital for success in chemistry.

Calculating Molar Mass

Now, molar mass is a physical property that represents the mass of a substance divided by the amount of that substance. We're going to say the SI unit for mass is kilograms, and the SI unit for the amount of a substance is moles. But molar mass is generally shown as being in units of grams per mole. And remember, when we see our shaded purple boxes, that means that's a definition or some type of formula you need to memorize. In this case, you need to memorize the molar mass formula. Molar mass itself equals grams per mole, and what you also need to realize is that molar mass is just one term that can talk about this relation of grams per mole. Besides molar mass, you might also hear it referred to as molar weight, molecular weight, or molecular mass. So just remember, all of these are talking about the same thing. They're all referring to the relationship of grams per mole, the mass of a substance divided by the amount of that substance.

Calculating Molar Mass Example 1

The example question says, calculate the molar mass of the compound NH₄₂SO₄. In order to do that, we're going to follow each of the following steps. Alright. So step 1, you have to count the number of each element within the given compound. Now if the elements are within parentheses, just remember to distribute the subscript to each element. Alright. So within these parentheses, we have NH₄. That means we have 1 nitrogen, 4 hydrogens, and this subscript of 2. What happens here is that 2 gets distributed, so it becomes 2 times 1, which will give us 2 nitrogens, and then we have 2 times 4, which will give us 8 hydrogens. We're done with everything in parentheses. Next, let's look at SO₄. So SO₄, there's only 1 sulfur there, so there we know that there's a one that we don't see, so there's just 1 sulfur. And then we see that there are 4 oxygens. So we've successfully counted each of these elements within the compound. Next, find the atomic masses of each element from the periodic table. So we have nitrogen, hydrogen, sulfur, and oxygen. Remember that the atomic mass is the number that is not a whole number, it usually has decimal places. That's because it's the average of all the isotopes for that particular element. So we have 14.01 grams per mole for nitrogen, 1.008 grams per mole for hydrogen, 32.07 grams per mole for sulfur, and 16 grams per mole for oxygen, roughly. The numbers on top are the atomic numbers, so let's not worry about those. Now, you're going to multiply together the number of each element with their atomic masses from the periodic table. So from step 1, we found out we had 2 nitrogens, 8 hydrogens, 1 sulfur, and 4 oxygens. Now multiplying them with their atomic masses gives us totals here. The new totals will be 28.02, 8.064, 32.07, and 64. Now that we have each of those totals, step 4 is you add up the totals after multiplication to determine the molar mass of the compound. So we take all these numbers and we add them all up together when we do that we're going to get a total of 132.154 grams per mole. So this would represent the molar mass of our compound. So these are the steps you must always use, always employ in order to determine the molar mass of any compound you come face to face with. So now that we know how to do that, let's continue onward with calculating molar mass.

Calculate the molecular weight of C₃H₅N₃O₃.

• A. 123.80 g/mol
• B. 131.10 g/mol
• C. 136.90 g/mol
• D. 147.10 g/mol

The reaction between nickel metal and hydrochloric acid is not a simple dissolution. The product formed is NiCl₂ • 6 H₂O (s), nickel (II) chloride hexahydrate, which has exactly 6 waters of hydration in the crystal lattice for every nickel ion. What is the molar mass of nickel (II) chloride hexahydrate, NiCl₂ • 6 H₂O (s)?

• A. 228.99 g/mol
• B. 231.01 g/mol
• C. 234.71 g/mol
• D. 237.69 g/mol

What is the molar mass of diazepam also known as Valium if 0.05570 mol weighs 15.86 g?

• A. 220.1 g/mol
• B. 241.4 g/mol
• C. 263.9 g/mol
• D. 284.7 g/mol

Here’s what students ask on this topic:

What is the formula for calculating molar mass?

The formula for calculating molar mass is the mass of a substance divided by the amount of that substance. It is typically expressed in grams per mole (g/mol). Mathematically, it can be represented as:

$M=\frac{m}{n}$

where $M$ is the molar mass, $m$ is the mass of the substance in grams, and $n$ is the amount of substance in moles. This formula is essential for various chemical calculations, including stoichiometry and determining reaction yields.

How do you calculate the molar mass of a compound?

To calculate the molar mass of a compound, follow these steps:

1. Identify the chemical formula of the compound.
2. Determine the atomic masses of each element in the compound from the periodic table.
3. Multiply the atomic mass of each element by the number of atoms of that element in the compound.
4. Add the total masses of all the elements together to get the molar mass of the compound.

For example, to calculate the molar mass of water (H₂O):

• Hydrogen (H) has an atomic mass of approximately 1 g/mol, and there are 2 hydrogen atoms.
• Oxygen (O) has an atomic mass of approximately 16 g/mol, and there is 1 oxygen atom.

So, the molar mass of H₂O is:

$2\times 1+16=18$ g/mol.

Why is molar mass important in chemistry?

Molar mass is crucial in chemistry because it allows chemists to convert between the mass of a substance and the amount of substance in moles. This conversion is essential for stoichiometric calculations, which involve determining the proportions of reactants and products in chemical reactions. Knowing the molar mass helps in calculating the yield of reactions, preparing solutions with precise concentrations, and understanding the properties of substances. Additionally, molar mass is fundamental in determining molecular formulas and understanding the composition of compounds. Overall, it is a key concept that underpins many aspects of chemical analysis and synthesis.

What is the difference between molar mass and molecular weight?

Molar mass and molecular weight are terms that are often used interchangeably, but they have subtle differences. Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is a physical property that relates the mass of a substance to the amount of substance. Molecular weight, on the other hand, is a dimensionless quantity that represents the sum of the atomic weights of all atoms in a molecule. While molar mass is used in practical calculations involving grams and moles, molecular weight is often used in theoretical contexts. Despite these differences, both terms ultimately describe the same concept: the mass of a given amount of substance.

How do you find the molar mass of an element?

To find the molar mass of an element, you need to look up its atomic mass on the periodic table. The atomic mass is usually listed below the element symbol and is expressed in atomic mass units (amu). For practical purposes in chemistry, the atomic mass in amu is numerically equivalent to the molar mass in grams per mole (g/mol). For example, the atomic mass of carbon (C) is approximately 12.01 amu, so the molar mass of carbon is 12.01 g/mol. This value represents the mass of one mole of carbon atoms.