Question 1

When should I use molarity versus ppm?

Accepted Answer

Use molarity (mol/L) for stoichiometric calculations, titrations, and reaction kinetics where you need to know the number of molecules. Use ppm for trace-level measurements such as water quality testing, contaminant analysis, or environmental monitoring where concentrations are very small.

Question 2

Why is solution density needed for concentration conversion?

Accepted Answer

Molarity is a volume-based unit (moles per liter) while wt% and ppm are mass-based (mass of solute per mass of solution). Density (g/mL) converts between mass and volume, making it essential for bridging these two types of units. For dilute aqueous solutions, density is approximately 1.0 g/mL, but for concentrated or non-aqueous solutions, the actual density must be used.