 # Concentration Units: Terms like “dilute” and “concentrated” are not specific. Percent by Mass: Mass % = mass of solute x 100 Total mass of solution Recall:

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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 KCl @ 40 o C? @ 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 225.0 g H 2 O? 25.0 g Na 2 SO 4 225.0 g H 2 O+ 25.0 g Na 2 SO 4 =25.0 250.0 = 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 = 0.300 mole NaOH 40.0 g 0.300 mol = 1.50 mol = 1.50 M NaOH 0.200 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 0.250 L = 0.050 mol 0.050 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 = 0.0506 mol 148.3 g = 0.0506 mol Mg(NO 3 ) 2 0.0250 L Molarity = Moles L = 2.02 M

Practice Problem How many grams of sodium chloride are needed to prepare 125 mL of a 0.037 M NaCl solution? Molarity (Moles/L) x volume (L) = Moles 0.037 M x 0.125 L = 0.0046 moles NaCl 0.0046 mol NaCl x 58.5 g= 0.27 g NaCl 1 mol

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