Mass Percent

Core Concept

Mass percent is the percentage of the total mass of a solution that is made up of the solute. Another term for this is percent composition. It is implied that percent composition is ALWAYS by mass.

  • Assume a 100-Gram Sample: When given only a chemical formula, assume you have exactly 100 grams to make the conversion between mass and percentage direct.

  • Account for All Atoms: Multiply the atomic mass of an element by its subscript in the formula before dividing by the total molar mass.

  • Double Check Your Units: Ensure the mass of the solute and the mass of the entire solution are in the same units (e.g., both in grams) before dividing.

  • Don't Forget the Solvent: When calculating mass percent for a solution, remember to add the mass of the solute to the mass of the solvent to find the true total mass.

Test Yourself

Assorted Multiple Choice
A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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Mass Percent

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Standard Mass Percent Calculation from Chemical Formula

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Using Mass Percent to Find Component Mass in a Sample

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Mass Percent in Hydrates and Complex Formulas

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Core Concept

Mass percent is the percentage of the total mass of a solution that is made up of the solute. Another term for this is percent composition. It is implied that percent composition is ALWAYS by mass.

  1. Formula: Mass percent is calculated by dividing the mass of the solute by the total mass of the solution and multiplying by 100.
    Mass percent = (mass of solute ÷ mass of solution) x 100

  2. Units: Mass percent is usually expressed as a percentage (%), but it can also be expressed in parts per million (ppm) or parts per billion (ppb).

  • Uses: Mass percent is commonly used in chemistry to express the concentration of a solution. It can also be used in other fields, such as in the food industry to express the fat content of a food product.

  • Limitations: Mass percent can be affected by changes in temperature and pressure, so it is important to use it at standard conditions. It also assumes that the solute and solvent are uniformly mixed throughout the solution, which may not always be the case.

  • Practice problems: To practice calculating mass percent, try solving problems that involve finding the mass percent of a solute in a solution given the mass of the solute and the total mass of the solution. Also, try solving problems that involve finding the mass of the solute in a solution given the mass percent and the total mass of the solution.

This systematic approach ensures that each organic compound has a unique and descriptive name, facilitating clear communication among chemists. For complex molecules, the naming can become more intricate, but these basic principles form the foundation of organic chemical nomenclature.

Example: Calculate the mass percent of each element in sodium sulfate, Na2​SO4​.

Step 1: Write the Formula and Determine the Atomic Masses

The formula for sodium sulfate is Na2​SO4​. It contains:

  • 2 sodium (Na) atoms

  • 1 sulfur (S) atom

  • 4 oxygen (O) atoms

Using the periodic table, we find the atomic masses of each element:

  • Sodium (Na): 22.99 g/mol

  • Sulfur (S): 32.07 g/mol

  • Oxygen (O): 16.00 g/mol

Step 2: Calculate the Molar Mass of Na2​SO4​

To find the molar mass of sodium sulfate, we add up the masses of all the atoms:

  • Sodium: 22.99 g/mol×2=45.98 g/mol

  • Sulfur: 32.07 g/mol×1=32.07 g/mol

  • Oxygen: 16.00 g/mol×4=64.00 g/mol

Adding these together:

  • Molar Mass of Na2​SO4​=45.98 g/mol+32.07 g/mol+64.00 g/mol=142.05 g/mol

Step 3: Calculate the Mass Percent of Each Element

To find the mass percent of each element, we use the formula: Mass Percent = (Mass of Element in 1 Mole / Molar Mass of Compound) x 100%

  1. Mass Percent of Sodium (Na): Mass Percent of Na = (45.98 g/mol / 142.05 g/mol) x 100% = 32.38%

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