How Do You Calculate the Formula Mass of a Compound?
To calculate the formula mass of a compound, you simply add up the atomic masses of all the atoms in its chemical formula. This is also sometimes referred to as formula weight or molecular weight (for covalent compounds).
The formula mass represents the total mass of all the atoms present in a single formula unit of a compound. It's crucial to remember that we use the average atomic mass for each element, as found on a periodic table. This accounts for the naturally occurring isotopes of each element.
Steps for Calculating Formula Mass:
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Identify the Chemical Formula: Start by writing down the correct chemical formula for the compound. For example, the formula for water is H₂O, and for sodium chloride is NaCl.
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Determine the Number of Atoms of Each Element: Count how many atoms of each element are present in the formula. In H₂O, there are two hydrogen atoms and one oxygen atom. In NaCl, there is one sodium atom and one chlorine atom.
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Find Atomic Masses: Consult a periodic table to find the average atomic mass of each element. These masses are usually given in atomic mass units (amu) or grams per mole (g/mol).
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Multiply and Sum: Multiply the atomic mass of each element by the number of atoms of that element in the formula. Then, add up all these products to obtain the formula mass.
Examples
Let's calculate the formula mass for a few compounds:
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Water (H₂O):
- Atomic mass of Hydrogen (H) ≈ 1.01 amu
- Atomic mass of Oxygen (O) ≈ 16.00 amu
- Formula mass = (2 × 1.01 amu) + (1 × 16.00 amu) = 18.02 amu
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Sodium Chloride (NaCl):
- Atomic mass of Sodium (Na) ≈ 22.99 amu
- Atomic mass of Chlorine (Cl) ≈ 35.45 amu
- Formula mass = (1 × 22.99 amu) + (1 × 35.45 amu) = 58.44 amu
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Sulfuric Acid (H₂SO₄):
- Atomic mass of Hydrogen (H) ≈ 1.01 amu
- Atomic mass of Sulfur (S) ≈ 32.07 amu
- Atomic mass of Oxygen (O) ≈ 16.00 amu
- Formula mass = (2 × 1.01 amu) + (1 × 32.07 amu) + (4 × 16.00 amu) = 98.09 amu
Important Note:
The term "molecular mass" is often used interchangeably with formula mass, but it's technically more appropriate for covalent compounds. For ionic compounds, the term "formula mass" is preferred because they don't exist as discrete molecules. However, the calculation method remains the same.
