How can the Molarity of a percentage solution be calculated?
Using 70% concentrated Nitric Acid as an example: 70% Nitric Acid means that 100 grams of this acid contains 70 grams of HNO3. The concentration is expressed at 70% wt./wt. or 70 wt. % HNO3. Some analysts prefer to work in acid concentration units of Molarity (moles/liter). To calculate the Molarity of a 70 wt. % Nitric Acid the number of moles of HNO3 present in 1 liter of acid needs to be calculated. Knowing the density of the acid, we can calculate the weight of 1L of 70% HNO3 to be 1400 grams. Knowing that the solution is 70 wt % would then allow the number of grams of HNO3 to be calculated: (0.700)(1400g) = 980.0 grams HNO3 per liter. Dividing the grams of HNO3 by the molecular weight of HNO3 (63.03 g/mole) gives the number of moles of HNO3 / L or Molarity which is 15.5 M. This theory explains the following equation used for calculating the Molarity of acids where the concentration of the acid is given in wt %: [(% x d) / MW] x 10 = Molarity; Where: % = Weight % of the acid; d = D
