Calculating Molar Mass: Videos & Practice Problems

Molar mass is a key 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 this relationship. The formula for molar mass is crucial for calculations in chemistry, linking mass and the amount of substance, which is fundamental in stoichiometry and various chemical reactions.

Calculating Molar Mass

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{\text{grams}}{\text{mole}}

It's important to note that molar mass is often referred to by various terms, including molar weight, molecular weight, or molecular mass. Regardless of the terminology used, all these terms describe 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 fields such as chemistry and material science, where precise measurements and calculations are essential for experiments and applications.

Calculating Molar Mass Example 1

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 we need to multiply the counts of the elements inside by 2. Thus, we have:

• 2 Nitrogens (N)
• 8 Hydrogens (H) (since \( 4 \times 2 = 8 \))

Next, for the sulfate ion \( \text{SO}_4 \), there is 1 sulfur (S) and 4 oxygens (O). Therefore, we summarize the counts as follows:

• 1 Sulfur (S)
• 4 Oxygens (O)

Now, we need to find the atomic masses of each element from the periodic table:

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

Next, 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: \( 4 \times 16.00 = 64.00 \, \text{g/mol} \)

Now, 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 calculate 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:

What is molar mass and how is it calculated?

Molar mass is a physical property that represents the mass of a substance divided by the amount of that substance, typically expressed in grams per mole (g/mol). To calculate molar mass, you sum the atomic masses of all the atoms in a molecule. For example, the molar mass of water (H₂O) is calculated by adding the atomic masses of 2 hydrogen atoms (2 x 1.01 g/mol) and 1 oxygen atom (16.00 g/mol), resulting in a molar mass of approximately 18.02 g/mol.

Why is molar mass important in chemistry?

Molar mass is crucial in chemistry because it links the mass of a substance to the amount of that substance in moles. This relationship is fundamental in stoichiometry, which involves calculating the quantities of reactants and products in chemical reactions. Understanding molar mass allows chemists to convert between mass and moles, facilitating the measurement and manipulation of substances in laboratory settings and ensuring accurate chemical reactions.

What is the difference between molar mass and molecular weight?

Molar mass and molecular weight are terms 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). Molecular weight, on the other hand, is a dimensionless quantity representing the sum of the atomic weights of all atoms in a molecule. Despite these differences, both terms refer to the same concept of the mass-to-mole relationship in a substance.

How do you find the molar mass of a compound?

To find 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) Sum these values to get the total molar mass. For example, for CO₂, the molar mass is calculated as (1 x 12.01 g/mol for C) + (2 x 16.00 g/mol for O) = 44.01 g/mol.

What units are used for molar mass?

The standard unit for molar mass is grams per mole (g/mol). This unit expresses the mass of one mole of a substance. While the SI unit for mass is kilograms, grams are more commonly used in chemistry for practical reasons. Therefore, when calculating or discussing molar mass, always use grams per mole to ensure consistency and accuracy in your measurements and calculations.