We will learn mass percent concentration as a way to express the concentration of an aqueous solution accurately. It is one of the most basic concepts in chemistry.
Basic Knowledge
Mass percent concentration [%] = (mass of solute ÷ total mass of solution) × 100
Total mass of solution = mass of solute + mass of solvent. For example, if 20g of table salt is dissolved in 80g of water, the solution has a mass of 100g, so the concentration is 20÷100×100=20%.
Three calculation patterns:
① Find the concentration: use the formula above as it is.
② Find the mass of the solute: solute = solution × concentration ÷ 100.
③ Find the amount of water (solvent) needed: solution - solute.
When an aqueous solution is diluted, the mass of the solute does not change. Adding water increases the denominator (the solution), so the concentration decreases.
Mass percent concentration [%] = (solute ÷ solution) × 100
Mass of solution = solute + solvent
Mass of solute = solution × concentration ÷ 100
Dilution（lowering concentration by adding solvent）
Concentration（increasing concentration by evaporating the solvent）
Volume percent concentration（studied in high school. Used for the alcohol concentration of alcoholic drinks）
Further Exploration (Background and Significance)
Concentration calculations are needed in every area of chemistry. The concept of concentration is essential in daily life, including the salt concentration of food, medication dosage, the concentration of intravenous solutions, seawater (about 3.4%), physiological saline (0.9%), and ethanol disinfectant (70-80%).
After middle school, molar concentration (mol/L) becomes the main method, but mass percent concentration is its foundation. Because many students overlook that “the denominator is the entire solution,” always remember that denominator = solute + solvent. A typical mistake is to say “20g of salt in 80g of water is 25%” instead of “20%.”
- Concentration = solute ÷ solution × 100
- The denominator is the entire solution (solute + solvent)
- Add water to dilute a solution
- Evaporate water to make a solution more concentrated
- The mass of the solute remains unchanged during dilution
- Seawater≈3.4%, physiological saline=0.9%
- Using the wrong denominator is the most common mistake
Points to Note (Easily Confused)
① The denominator is the entire solution (= solute + solvent). Do not use only the solvent as the denominator. ② The mass of the solute remains unchanged during dilution. The total solution and concentration change. ③ Diluting means adding water (the solute remains unchanged), while concentrating means evaporating water. ④ Do not forget to multiply by ×100. Write 20%, not the ratio 0.2.
Practice
- What is the mass percent concentration when 15g of table salt is dissolved in 185g of water?
- How many grams of table salt are contained in 200g of 5% salt water?
- If 100g of 10% salt water is mixed with 100g of water, what is the concentration of the resulting salt water?