To determine the percent of oxygen in a compound, you calculate its mass percent composition by comparing the total mass of oxygen within the compound to the total mass of the compound itself.
Understanding Mass Percent Composition
Mass percent composition refers to the percentage by mass of each element in a given chemical compound. It's a crucial concept in chemistry for understanding the elemental makeup of substances, which can be useful in various applications from quality control to designing chemical reactions.
Step-by-Step Calculation for Oxygen
Finding the mass percent of oxygen involves a straightforward calculation. The general formula for calculating the mass percent of any element in a compound is:
Formula for Mass Percent
$$
\text{Mass % of Element} = \left( \frac{\text{Total mass of element in compound}}{\text{Molar mass of compound}} \right) \times 100\%
$$
Practical Steps
To apply this formula specifically for oxygen, follow these steps:
- Determine the Total Mass of Oxygen in the Compound:
- Find the atomic mass of oxygen from the periodic table. The atomic mass of oxygen is approximately 16.00 g/mol.
- If the chemical formula of the compound indicates multiple oxygen atoms (e.g., O₂, SO₃, CaCO₃), multiply the atomic mass of oxygen by the number of oxygen atoms present in one molecule or formula unit of the compound. For instance, in SO₃, there are three oxygen atoms, so the total mass contributed by oxygen would be 3 × 16.00 g/mol.
- Calculate the Molar Mass of the Compound:
- Find the atomic masses of all other elements present in the compound from the periodic table.
- Multiply each element's atomic mass by the number of atoms of that element in the compound's formula.
- Sum the masses of all individual elements to obtain the compound's total molar mass. This value represents the mass of one mole of the compound.
- Divide and Multiply:
- Divide the total mass of oxygen (calculated in Step 1) by the molar mass of the compound (calculated in Step 2).
- Multiply the result by 100% to express the value as a percentage. The units will cancel out, leaving a pure percentage.
Example: Finding the Percent of Oxygen in Water (H₂O)
Let's illustrate this with a common compound, water (H₂O).
| Step | Description | Calculation |
|---|---|---|
| 1. | Total Mass of Oxygen | Atomic mass of O = 16.00 g/mol In H₂O, there is 1 oxygen atom. Total mass of O = 1 × 16.00 g/mol = 16.00 g/mol |
| 2. | Molar Mass of Water (H₂O) | Atomic mass of H = 1.01 g/mol In H₂O, there are 2 hydrogen atoms. Total mass of H = 2 × 1.01 g/mol = 2.02 g/mol Molar mass of H₂O = (2.02 g/mol for H) + (16.00 g/mol for O) = 18.02 g/mol |
| 3. | Calculate Percent Oxygen | $$ \text{Mass % of Oxygen} = \left( \frac{16.00 \text{ g/mol}}{18.02 \text{ g/mol}} \right) \times 100\% $$ Mass % of Oxygen ≈ 88.79% |
Therefore, water is approximately 88.79% oxygen by mass.
Why is this important?
Understanding how to calculate the percent composition of elements, including oxygen, is fundamental for:
- Verifying Purity: Confirming the composition of synthesized compounds.
- Stoichiometry: Predicting the amount of reactants and products in chemical reactions.
- Material Science: Designing materials with specific properties based on their elemental makeup.
- Environmental Analysis: Assessing the concentration of elements in various samples.
