Question

What are the differences between m and M?

Answer 1

Both m and M are units of the concentration of a chemical solution. The lower case m indicates molality, which is calculated using moles of solute per kilograms of solvent. A solution using these units is called a molal solution (e.g., 0,1 m NaOH is a 0.1 molal solution of sodium hydroxide).
Upper case M is molarity, which is moles of solute per litre of solution (not solvent). A solution using this unit is termed a molar solution (e.g., 0.1 M NaCl is a 0.1 molar solution of sodium chloride).

Molality (m) = moles solute / kilogram solvent
The units of molality are mol/kg.

Molarity (M) = moles solute / liters solution
The units of molarity are mol/L.