Molaridade a partir de massa e MM

Molarity from mass and molar weight

Molarity (M, mol/L) tells you how much solute sits in each liter of solution. When you start from a mass, the route is M = (m / MW) / V. Here m is the solute mass in grams, MW the molar weight in g/mol, and V the volume of the solution in liters. Say you dissolve 5.85 g of NaCl (MW = 58.5 g/mol) in 1 L: that works out to 5.85/58.5 = 0.1 mol, so 0.1 M. One thing to watch is molality (mol of solute per kg of solvent), which doesn't shift with temperature. Molarity does drift a little as the solvent expands when it warms up.

Applications

You'll see it when preparing standard solutions in analytical chemistry, in clinical labs (electrolytes reported in mEq/L come from molarity), in agrochemistry for pesticide dosing, and in industrial brines or isotonic solutions like saline 0.9% and glucose 5%. Any work involving stoichiometry, titration, or reaction kinetics in mol/L leans on it.

FAQ

Why use moles instead of grams? Reactions happen in fixed mole ratios. 1 mol of NaOH neutralizes 1 mol of HCl every time, no matter what the two masses happen to be.

Volume of solution or solvent? Use the total volume of the solution after everything has dissolved, not the volume of pure solvent you poured in.

Molarity vs molality? Molarity is mol/L of solution and depends on T. Molality is mol/kg of solvent, stays put regardless of T, and shows up in colligative properties.