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).

Example: Calculate the mass percent of each element in sodium sulfate, Na2SO4\text{Na}_2\text{SO}_4Na2​SO4​.

Step 1: Write the Formula and Determine the Atomic Masses

The formula for sodium sulfate is Na2SO4\text{Na}_2\text{SO}_4Na2​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 Na2SO4\text{Na}_2\text{SO}_4Na2​SO4​

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

1. Sodium: 22.99 g/mol×2=45.98 g/mol22.99 \, \text{g/mol} \times 2 = 45.98 \, \text{g/mol}22.99g/mol×2=45.98g/mol

2. Sulfur: 32.07 g/mol×1=32.07 g/mol32.07 \, \text{g/mol} \times 1 = 32.07 \, \text{g/mol}32.07g/mol×1=32.07g/mol

3. Oxygen: 16.00 g/mol×4=64.00 g/mol16.00 \, \text{g/mol} \times 4 = 64.00 \, \text{g/mol}16.00g/mol×4=64.00g/mol

Adding these together:

Molar Mass of Na2SO4=45.98 g/mol+32.07 g/mol+64.00 g/mol=142.05 g/mol\text{Molar Mass of Na}_2\text{SO}_4 = 45.98 \, \text{g/mol} + 32.07 \, \text{g/mol} + 64.00 \, \text{g/mol} = 142.05 \, \text{g/mol}Molar Mass of Na2​SO4​=45.98g/mol+32.07g/mol+64.00g/mol=142.05g/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 MoleMolar Mass of Compound)×100%\text{Mass Percent} = \left( \frac{\text{Mass of Element in 1 Mole}}{\text{Molar Mass of Compound}} \right) \times 100\%Mass Percent=(Molar Mass of CompoundMass of Element in 1 Mole​)×100%

1. Mass Percent of Sodium (Na):

Mass Percent of Na=(45.98 g/mol142.05 g/mol)×100%=32.38%\text{Mass Percent of Na} = \left( \frac{45.98 \, \text{g/mol}}{142.05 \, \text{g/mol}} \right) \times 100\% = 32.38\%Mass Percent of Na=(142.05g/mol45.98g/mol​)×100%=32.38%

1. 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.

2. 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.

3. 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.