To calculate the molecular mass of benzene (C₆H₆), you sum the atomic masses of all the atoms present in its chemical formula. The exact molecular mass of benzene is approximately 78.11 g/mol.
Understanding Molecular Mass
Molecular mass (often referred to as molar mass when expressed in grams per mole, g/mol) is a fundamental chemical property that represents the sum of the atomic masses of all atoms in a molecule. It's a crucial value for various chemical calculations, including stoichiometry, solution preparation, and understanding chemical reactions.
Benzene is an organic compound with the chemical formula C₆H₆. This formula indicates that each benzene molecule consists of six carbon (C) atoms and six hydrogen (H) atoms.
Step-by-Step Calculation of Benzene's Molecular Mass
Calculating the molecular mass involves identifying the atomic mass of each element in the compound, multiplying it by the number of times that element appears in the molecule, and then summing these individual contributions.
Here’s how to calculate the molecular mass for benzene (C₆H₆):
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Identify the elements and their quantities:
- Carbon (C): There are 6 carbon atoms in a benzene molecule.
- Hydrogen (H): There are 6 hydrogen atoms in a benzene molecule.
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Determine the atomic mass (Ar) of each element:
- The approximate atomic mass of Carbon (C) is typically used as 12 atomic mass units (amu) for quick calculations, though its more precise value is around 12.011 amu.
- The approximate atomic mass of Hydrogen (H) is typically used as 1 amu for quick calculations, with a more precise value around 1.008 amu.
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Apply the formula for molecular mass (Mr):
The general formula for calculating the molecular mass of benzene is:
Mr (C₆H₆) = (Number of C atoms × Ar of C) + (Number of H atoms × Ar of H) -
Substitute the values and perform the calculation:
Using the commonly rounded atomic masses:
Mr (C₆H₆) = (6 × 12) + (6 × 1)
Mr (C₆H₆) = 72 + 6
Mr (C₆H₆) = 78 g/molFor a more precise result, using more accurate atomic masses:
Mr (C₆H₆) = (6 × 12.011) + (6 × 1.008)
Mr (C₆H₆) = 72.066 + 6.048
Mr (C₆H₆) = 78.114 g/molTherefore, the commonly accepted molecular mass of benzene, often rounded to two decimal places for practical use, is 78.11 g/mol.
Breakdown of Atomic Contributions
The table below summarizes the contribution of each element to the total molecular mass of benzene, using rounded atomic masses for clarity:
| Element | Number of Atoms | Atomic Mass (Ar) (amu) | Total Mass Contribution (amu) |
|---|---|---|---|
| Carbon (C) | 6 | 12 | 6 × 12 = 72 |
| Hydrogen (H) | 6 | 1 | 6 × 1 = 6 |
| Total | 78 |
Importance of Knowing Molecular Mass
Understanding the molecular mass of a compound like benzene is essential for several reasons:
- Stoichiometric Calculations: It allows chemists to accurately convert between the mass of a substance and the number of moles, which is critical for balancing chemical equations and calculating reactant or product quantities.
- Solution Preparation: When preparing solutions of specific concentrations (e.g., molarity), knowing the molecular mass helps determine the precise mass of solute needed.
- Chemical Analysis: Molecular mass data is often used in analytical techniques, such as mass spectrometry, to identify unknown compounds or verify the composition of known ones.
- Industrial Processes: In chemical manufacturing and research, accurate molecular mass data is vital for process design, quality control, and calculating material yields.
Tips for Accurate Molecular Mass Determinations
- Consult a Reliable Periodic Table: Always refer to a reliable periodic table of elements for the most up-to-date and precise atomic masses, especially when high accuracy is required.
- Verify the Chemical Formula: A common source of error is using an incorrect chemical formula. Always double-check the formula before starting calculations.
- Include Appropriate Units: Molecular mass is typically expressed in atomic mass units (amu) for a single molecule, or grams per mole (g/mol) when referring to the molar mass of a macroscopic sample.
By following these guidelines, you can accurately calculate the molecular mass of benzene and confidently apply this knowledge in various chemical contexts.
