Calculate the molecular weight of the solute in the solution. For example, assume the solute is sodium chloride, which has the molecular formula NaCl. To find the molecular weight of sodium chloride, add the weight of Na (22.99) to Cl (35.45), which equals 58.44 grams per mole.
To convert milligrams per liter to molarity, use this formula: Molarity = mg/L / (molecular weight of solute) x 1,000. If we assume that the solute in the solution is sodium chloride and the concentration is 0.5 mg/L, the equation would be Molarity = 0.5 mg/L / (58.44) x 1,000. The answer is 8.556 micromoles (μM).
Convert from micromoles to moles or millimoles if necessary. To calculate moles, use the equation Moles = micromoles / 1,000,000. To calculate millimoles, use the equation Millimoles = micromoles / 1,000.