Concentration Units
Percent, ppm and molarity express concentration.
About this topic
Concentration can be expressed in several ways. Molarity (mol/L) is moles of solute per litre of solution. Molality (mol/kg) is moles of solute per kilogram of solvent. Mass percent is (mass of solute ÷ mass of solution) × 100. Mole fraction is moles of one component divided by total moles, and parts per million (ppm) is used for traces.
Choose by purpose: molarity is convenient for reactions and titrations, but changes slightly with temperature because volume expands. Molality does not, so it is used in colligative property calculations. Mass percent is common on labels, and ppm for pollutants, where 1 ppm is about 1 mg per litre of water. Converting between them often needs the solution density.
Worked example: dissolving 10.0 g of NaCl in 90.0 g of water gives a solution of 100.0 g, so the mass percent is 10.0 percent. The moles of NaCl are 0.171, so molality is 0.171 / 0.0900 = 1.90 m. Drinking water standards might allow 10 ppm of nitrate, and household bleach is about 5 percent sodium hypochlorite.
Students commonly mix up molarity and molality or divide by the volume of solvent instead of solution. Check which denominator you need. Mole fractions of all components sum to 1.
Key ideas
- Mass percent
- Parts per million
- Molality for temperature changes
Quick check
Use cases: where Concentration Units helps
- Homework: review the key idea behind Concentration Units before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Concentration Units without notes.
- Visual learning: play with the interactive lab to see how Concentration Units changes when you adjust its inputs.
- Teaching a friend: explain Concentration Units in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Concentration Units?
Percent, ppm and molarity express concentration.
What will I learn in Concentration Units?
Mass percent. Parts per million. Molality for temperature changes.
Is Concentration Units part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Concentration Units lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Concentration Units?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.