Concentration Units: Terms like “dilute” and “concentrated” are not specific. Percent by Mass: Mass % = mass of solute x 100 Total mass of solution Recall: Total mass of solution = mass of solute + mass of solvent Example: 20 g of salt in 60 g of water = 20/80 = 25%
What is the mass % of a saturated solution of 40 o 40 o C the solubility of KCl is ? Mass % = ? 40g / 100g water = 40 / 140 = 28.6 %
Practice Problem What is the mass percent of Na 2 SO 4 in a solution made by dissolving 25.0 g Na 2 SO 4 in g H 2 O? 25.0 g Na 2 SO g H 2 O g Na 2 SO 4 = = 10.0 %
Molarity (M) = moles of solute Liter of solution Example: 12.0 g NaOH into 200. ml of solution 12.0 g x 1 mol = mole NaOH 40.0 g mol = 1.50 mol = 1.50 M NaOH L L
Example: How many grams of NaOH are needed to make 250. ml of 0.20 M solution? Molarity (M) = moles of solute Liter of solution Molarity x liters = moles 0.20 mol / L x L = mol mol x 40.0 g /mol = 2.0 g NaOH
Practice Problem What is the molarity of a solution made by dissolving 7.50 g of magnesium nitrate, Mg(NO 3 ) 2, in enough water to make 25.0 mL of solution? 7.50 g Mg(NO 3 ) 2 x 1 mol = mol g = mol Mg(NO 3 ) L Molarity = Moles L = 2.02 M
Practice Problem How many grams of sodium chloride are needed to prepare 125 mL of a M NaCl solution? Molarity (Moles/L) x volume (L) = Moles M x L = moles NaCl mol NaCl x 58.5 g= 0.27 g NaCl 1 mol