Calculating Molar Mass: Videos & Practice Problems

Molar mass, also known as molecular weight or molecular mass, is a critical concept in chemistry that refers to the mass of a substance divided by the amount of that substance. It is typically expressed in units of grams per mole (g/mol). This relationship is essential for understanding the composition of compounds and performing stoichiometry calculations. The SI unit for mass is kilograms (kg), while the amount of a substance is measured in moles. For practical purposes in chemistry, molar mass allows chemists to quantify and relate the mass of different substances on a molecular level, facilitating the comparison and reaction of different compounds. Understanding molar mass is fundamental for students to grasp the principles of chemical reactions and the quantitative aspects of substances in chemistry.

Molar Mass Concept

Molar mass is a crucial physical property that quantifies the mass of a substance relative to the amount of that substance. In scientific terms, the SI unit for mass is kilograms, while the SI unit for the amount of substance is moles. However, molar mass is typically expressed in grams per mole (g/mol). It is essential to memorize the formula for molar mass, which can be stated as:

Molar Mass = &frac{grams}{moles}

It's important to note that molar mass is often synonymous with several other terms, including molar weight, molecular weight, and molecular mass. Regardless of the terminology used, all these terms refer to the same concept: the relationship of grams per mole, which represents the mass of a substance divided by the amount of that substance. Understanding this relationship is fundamental in various fields of chemistry, as it allows for the conversion between mass and moles, facilitating calculations in stoichiometry and other chemical analyses.

Molar Mass Calculation Example

To calculate the molar mass of the compound \( \text{NH}_4\text{(SO}_4\text{)}_2 \), follow these systematic steps:

First, identify and count the number of each element in the compound. For the ammonium ion \( \text{NH}_4 \), there is 1 nitrogen (N) and 4 hydrogens (H). The subscript 2 outside the parentheses indicates that both nitrogen and hydrogen must be multiplied by 2, resulting in 2 nitrogens and 8 hydrogens. Next, for the sulfate ion \( \text{SO}_4 \), there is 1 sulfur (S) and 4 oxygens (O). Thus, the total counts are:

• Nitrogen: 2
• Hydrogen: 8
• Sulfur: 1
• Oxygen: 4

Next, refer to the periodic table to find the atomic masses of each element:

• Nitrogen (N): 14.01 g/mol
• Hydrogen (H): 1.008 g/mol
• Sulfur (S): 32.07 g/mol
• Oxygen (O): 16.00 g/mol

Now, multiply the number of each element by its atomic mass:

• For Nitrogen: \( 2 \times 14.01 = 28.02 \, \text{g/mol} \)
• For Hydrogen: \( 8 \times 1.008 = 8.064 \, \text{g/mol} \)
• For Sulfur: \( 1 \times 32.07 = 32.07 \, \text{g/mol} \)
• For Oxygen: \( 4 \times 16.00 = 64.00 \, \text{g/mol} \)

Finally, add these totals together to find the molar mass of the compound:

\[\text{Total Molar Mass} = 28.02 + 8.064 + 32.07 + 64.00 = 132.154 \, \text{g/mol}\]

Thus, the molar mass of \( \text{NH}_4\text{(SO}_4\text{)}_2 \) is approximately 132.15 g/mol. This method can be applied to determine the molar mass of any compound you encounter.

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

Answer: (B)

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

Answer: (D)

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

Answer: (D)

Here’s what students ask on this topic:

How can one find molar mass?

To find the molar mass of a compound, you'll need to look at the chemical formula of the substance and use the periodic table of elements. Each element listed in the chemical formula has an atomic mass, which is given in atomic mass units (amu) or grams per mole (g/mol) on the periodic table.

Here's the step-by-step process:

1. Write down the chemical formula of the compound.
2. Identify each element in the formula.
3. For each element, find its atomic mass on the periodic table.
4. Multiply the atomic mass of each element by the number of atoms of that element in the compound.
5. Add up all the products from step 4 to get the total molar mass of the compound.

For example, for water (H₂O):

• Hydrogen (H) has an atomic mass of approximately 1.01 g/mol, and there are 2 atoms of hydrogen.
• Oxygen (O) has an atomic mass of approximately 16.00 g/mol, and there is 1 atom of oxygen.

So, the molar mass of water would be:

( 2 atoms H × 1.01 g/mol ) + ( 1 atom O × 16.00 g/mol ) = 2.02 g/mol + 16.00 g/mol = 18.02 g/mol

How do you calculate molar mass?

Calculating molar mass is a straightforward process. The molar mass of a substance is the mass of one mole of that substance, and it is expressed in grams per mole (g/mol). To calculate it, you'll need the periodic table of elements for reference.

Here's how you do it:

1. Write down the chemical formula of the substance whose molar mass you want to calculate. For example, if you're calculating the molar mass of water (H2O), you note that it consists of 2 hydrogen (H) atoms and 1 oxygen (O) atom.
2. Look up the atomic mass of each element in the chemical formula on the periodic table. The atomic mass is usually located under the element symbol. For H2O, hydrogen has an atomic mass of about 1.01, and oxygen has an atomic mass of about 16.00.
3. Multiply the atomic mass of each element by the number of atoms of that element in the molecule. For water, you would do 2(1.01) for hydrogen and 1(16.00) for oxygen.
4. Add these values together to get the total molar mass. For water, the calculation is 2(1.01)+1(16.00)=2.02+16.00=18.02.

What is molar mass?

Molar mass is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of substance. The unit for molar mass is grams per mole (g/mol). It tells us how much one mole of a substance weighs. A mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as the amount of any substance that contains as many elementary entities (e.g. atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (C12), the isotope of carbon with relative atomic mass 12 by definition. This number is known as Avogadro's number and is approximately 6.022×1023. Molar mass is calculated by summing the total mass of the atomic masses of the elements in a compound, based on the periodic table, multiplied by the number of atoms of each element in the molecular formula.

How do you find the molar mass of an element?

To find the molar mass of an element, you'll need to look at the periodic table of elements. Each element on the periodic table has an atomic weight listed, which is usually found beneath the chemical symbol. This atomic weight, or relative atomic mass, is the average mass of atoms of an element, measured in atomic mass units (amu), and it takes into account the various isotopes and their abundances.

The molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). For elements, the molar mass in grams per mole is numerically equal to the atomic weight in atomic mass units. For example, if the atomic weight of carbon is 12.01 amu, then the molar mass of carbon is 12.01 g/mol.

To calculate the molar mass of an element, simply use the atomic weight from the periodic table and express it in grams per mole. This is the molar mass of that element.

How can the molar mass of a compound be found?

To find the molar mass of a compound, you need to calculate the sum of the molar masses of all the atoms in a single molecule of that compound. Here's how you do it:

1. Write down the chemical formula of the compound.
2. Identify each element present in the compound and the number of atoms of each element.
3. Look up the atomic mass of each element on the periodic table. The atomic mass is usually listed below the element symbol and represents the mass of one mole of atoms of that element, expressed in atomic mass units (amu) or grams per mole (g/mol).
4. Multiply the atomic mass of each element by the number of atoms of that element in the compound.
5. Add up all the values from step 4 to get the total molar mass of the compound.

For example, for water (H₂O), you would take the atomic mass of hydrogen (approximately 1.01 g/mol), multiply it by 2 for the two hydrogen atoms, and then add the atomic mass of oxygen (approximately 16.00 g/mol). The molar mass of water would be:

( 2 × 1.01 ) + 16.00 = 18.02 g/mol