As a chemistry expert, I can tell you that the "big M" in chemistry often refers to molarity, which is a measure of the concentration of a solute in a solution. Molarity is expressed as moles of solute per liter of solution (mol/L). It's a crucial concept in chemistry, particularly in stoichiometry and the calculation of reaction yields.

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Both m and M are units of the concentration of a chemical solution. ... Upper case M is molarity, which is moles of solute per liter 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).read more >>