Molar Mass

Core Concept

The molar mass of a molecule or ionic compound is the sum of the molar masses of all the atoms present in its chemical formula.

Step 1: List each element present in the compound.
Step 2: Determine the number of atoms of each element using the subscripts in the formula.
Step 3: Multiply the number of atoms by the molar mass of that element (found on the periodic table).
Step 4: Add the results together.

Practice Tips

Account for Subscripts: Always multiply the element's molar mass by its subscript in the formula to ensure every atom is counted.
Handle Parentheses Carefully: Multiply the mass of every element inside a set of parentheses by the subscript located outside those parentheses.
Watch Your Units: Consistently write $g/mol$ after your calculations to distinguish molar mass from simple atomic mass units ($amu$).
Verify Significant Figures: Match the precision of your final molar mass to the number of decimal places provided by your periodic table or the given data.

Test Yourself

Assorted Multiple Choice

To find the molar mass of a compound like calcium carbonate, what is the first step you must take using the periodic table?

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Core Concept

Molar mass is the mass of one mole of a substance. It is usually expressed in grams per mole (g/mol).

Calculating Molar Mass:

To find a substance's molar mass, you add up the atomic masses of all the atoms in one of its molecules. Here's an example for calculating the molar mass of sulfuric acid (H2SO4).

The formula for sulfuric acid is H2SO4, which contains two hydrogen (H) atoms, one sulfur (S) atom, and four oxygen (O) atoms.

To find the total molar mass, follow these steps:

Find the atomic masses of each element:
- Hydrogen (H): 1.01 g/mol
- Sulfur (S): 32.07 g/mol
- Oxygen (O): 16.00 g/mol
Multiply each atomic mass by the number of atoms in the formula:
- Hydrogen: (1.01 g/mol) × 2 = 2.02 g/mol
- Sulfur: (32.07 g/mol) × 1 = 32.07 g/mol
- Oxygen: (16.00 g/mol) × 4 = 64.00 g/mol
Add the results together to find the total molar mass:
- Molar Mass of H2SO4 =2.02 g/mol + 32.07 g/mol + 64.00 g/mol = 98.09 g/mol

The molar mass of sulfuric acid (H2SO4) is 98.09 g/mol.

Core Concept

Atomic Mass vs. Molar Mass: On the periodic table, the average atomic mass of an element (measured in atomic mass units, or amu) is numerically identical to its molar mass (measured in grams per mole, or g/mol).

Example: One atom of Carbon has a mass of roughly 12.01 amu. Therefore, one mole of Carbon atoms has a mass of 12.01 grams.

Brain Hack Tip

🧠 Here is a tip of how to remember how to remember how to name acids 🧠

Remember to “mole-tiply”

When converting to moles to grams using molar mass - you will multiply the moles by molar mass to get to grams.

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