Solutions and Molarity 4-20-09

What is a Solution? A Solution is a homogeneous mixture composed of two or more substances. Example: Salt Water (Fig 13-2, pg 469) NaCl(s)  Na(aq) + Cl(aq) H2O

Solutions Consist of Two Things Solvent is the substance in which the solute dissolves to make the solution. (Example: Water) Solute is the substance dissolved in a solution. (Example: Salt) Solutions are often called Aqueous. (A solution in which the solvent is H2O.)

Expressing Concentrations The quantity of solute in a specific quantity of solvent is a measurement of the Concentration of the solution. Concentration is measured in units of Molarity. Molarity is the ratio of solute amount (in moles) to solution volume (in liters).

Molarity Molarity = M = What is the Molarity of a solution that has a volume of 1.5L and contains 3.5 mols of NaCl? moles of solute liters of solution mol L 3.5 mols 1.5 L = 2.33 M

Sample Problem #1 What is the Molarity of a solution that has a volume of 2.5L and contains 4.3 mols of NaCl? But what if the question involved mL instead of L? 4.3 mols 2.5 L = 1.72 M

Example (mL to L) What is the Molarity of a solution that has a volume of 400mL and contains 5.2 mols of NaCl? Now you try one! 5.2 mols | 400 mL | 1000 mL | 1 L | = 13 M

Sample Problem #2 What is the Molarity of a solution that has a volume of 250mL and contains 3.8 mols of NaCl? But what if the question involved grams instead of mols? 3.8 mols | 250 mL | 1000 mL | 1 L | = 15.2 M

Example (grams to mols) What is the Molarity of a solution that has a volume of 1.6L and contains 60 grams of NaCl? Now you try one! 60 g NaCl | 1.6 L | 1 mol NaCl | 58.45 g NaCl | = 0.64 M

Sample Problem #3 What is the Molarity of a solution that has a volume of 2.3L and contains 95 grams of NaCl? But what if you were just given grams and mL in the question? 95 g NaCl | 2.3 L | 1 mol NaCl | 58.45 g NaCl | = 0.71 M

Example (mL to L & grams to mols) What is the Molarity of a solution that has a volume of 600 mL and contains 75 grams of NaCl? Now you try one! 75 g NaCl | 600 mL | 1000 mL | 1 L | 1 mol NaCl | 58.45 g NaCl | = 2.14 M

Sample Problem #4 What is the Molarity of a solution that has a volume of 900 mL and contains 100 grams of NaCl? Now what if I wanted to make my own Molarity solution? 100 g NaCl | 900 mL | 1000 mL | 1 L | 1 mol NaCl | 58.45 g NaCl | = 1.90 M

Example (Solute from Molarity) How many grams of NaCl should be added to make a 100mL flask of a 3.5 M solution? Now you try one! mol NaCl 100mL | | 1 L | 1000 mL | 3.5 | 1 L | 58.45 g NaCl| 1 mol NaCl| = 20.46 g of NaCl

Sample Problem #5 How many grams of NaCl should be added to make a 500mL flask of a 5.4 M solution? Can I dilute a Molarity sol’n to make a new one? mol NaCl 500mL | | 1 L | 1000 mL | 5.4 | 1 L | 58.45 g NaCl| 1 mol NaCl| = 157.82 g of NaCl

Example of Dilutions You obtain 50 mL of a 4 M solution of NaCl. How many mL are in the final volume of a 2 M solution of NaCl? M1V1 = M2V2 (4M) (50mL) = (2M) (?mL) (200) = (2) (?mL) Divide by 2 on both sides 100 = mL

Sample Problem #6 You obtain 75 mL of a 5 M solution of NaCl. How many mL are in the final volume of a 4 M solution of NaCl? M1V1 = M2V2 (5M) (75mL) = (4M) (?mL) (375) = (4) (?mL) Divide by 4 on both sides 93.75 = mL