How Do You Calculate Concentration?
Concentration describes how much solute is dissolved in a given amount of solvent or solution. Different units—molarity, PPM, percentages—are used depending on the context and precision needed.
Concentration is calculated by dividing the amount of solute by the amount of solvent or solution. Common formulas: M = n/V (molarity), PPM = mg/L, %w/w = (g solute / g solution)×100, %v/v = (mL solute / mL solution)×100.
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Step-by-step worked examples
What is the weight percent of 15 g NaCl dissolved in 150 g of solution?
%w/w = (mass solute / mass solution) × 100 %w/w = (15 / 150) × 100 %w/w = 10%
A solution contains 250 mg of sugar in 1 L. What is the PPM?
PPM = mass (mg) / volume (L) PPM = 250 mg / 1 L PPM = 250 mg/L
Volume percent of 20 mL ethanol in 200 mL total solution?
%v/v = (volume solute / volume solution) × 100 %v/v = (20 / 200) × 100 %v/v = 10%
Flashcards
Quick quiz
Q1.10 g salt in 100 g solution — what is %w/w?
Q2.What unit is best for measuring pollutants in water?
Q3.50 mL water + 50 mL alcohol = 100 mL. What is %v/v alcohol?
Q4.Which formula uses mass of solute and mass of solution?
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Common mistakes
Using volume of solute for %w/w (weight percent). — Correct: %w/w uses mass (grams) of both solute and solution, never volumes.
Forgetting to multiply by 100 when calculating percentages. — Correct: Always multiply the decimal ratio by 100 to get a true percentage.
Confusing PPM (mg/L) with molarity. — Correct: PPM uses mass (mg) and volume (L) directly; molarity uses moles and volume (L).
Using total volume for %v/v when solute volume is unknown. — Correct: %v/v requires both solute volume and final solution volume; if they're not additive, the calculation may be invalid.
FAQ
What is the difference between %w/w, %v/v and PPM?
%w/w is mass percent, %v/v is volume percent, PPM is parts per million (mg/L). Use the one that matches your solute/solvent states.
How do I convert molarity to %w/w?
Multiply molarity by molar mass to get g/L, then divide by solution density (g/mL) and multiply by 100.
Why is PPM used for pollutants?
Because contaminants are often present in tiny amounts (μg/L or mg/L), making PPM a natural unit.
Can %v/v and %w/w be the same number?
Rarely — only if the solute and solvent densities are identical, which is almost never true.