4.5 Concentration of Solutions

1. Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. Molarity is one way to measure the concentration of a solution. moles of solute Molarity (M) = volume of solution in liters 4.5 Concentration of Solutions

2. Molarity Calculations Expressing the concentration of an Electrolyte

3. Interconverting Molarity, Moles and Volume • The definition of molarity contains three quantities: molarity (M), moles of solute, and liters of solution. • To solve for moles, multiply liters of solution X molarity • Notice that the units for volume cancel out • To solve for volume of solution, divide moles of solute by the molarity • Notice that the units for moles cancel out

4. Preparation of a Solution

5. Dilution Mconc x Vconc = Mdilute x Vdilute How many mL of 1.00 M HCl must be diluted to make 350 mL of 0.250 M HCl solution? How many mL 2.50 M NaOH must be diluted to make 0.500 L of 0.750 M NaOH solution?

6. 4.6 Solution Stoichiometry and Chemical Analysis

7. In stoichiometric calculations, if you know the quantity of one reactant (or product) in a balanced chemical reaction, you can calculate the quantities of other reactants and products. • When working with solutions of known molarity, we use molarity and volume to determine the number of moles. Moles of solute = M x L soln • For example: how many grams of Ca(OH)2 are required to neutralize 25.0 mL of 0.100 M HN03? (See sample exercise 4.15. Also, try solving the practice exercises to see if you can get the posted answers.) Ca(OH)2 + 2 HNO3 2 H2O(l) + Ca(NO3)2 • Calculate number of moles of HNO3 Moles = Liters x M • Use moles of HNO3 and mole ratio to solve for grams of Ca(OH)2 grams Ca(OH)2 = moles HNO3 x mole ratio x molar mass of Ca(OH)2

8. Titration • The analytical technique in which one can calculate the concentration of a solute in a solution. Titrations can be conducted for acid-base reactions, precipitation, or redox reactions. • An acid-base titration is shown below. The chemist is trying to determine the concentration of an acid solution. • Titrations will be covered more thoroughly in Ch 17.

9. The sample of the solution of unknown concentration is combined with a reagent solution of known concentration, called a standard solution. • In the acid-base titration example shown, a standard NaOH solution is added to a specific volume of the acid solution of unknown concentration until the neutralization reaction between the HCl and the NaOH is complete. • The point at which stoichiometrically equivalent quantities are combined is known as the equivalence point of the titration. • How does one know the equivalence point has been reached? Acid-base titrations use dyes known as indicators. An indicator called phenolphthalein (phph) is colorless in an acidic solution but pink in a basic solution. • NaOH is added drop by drop to the acid solution containing phph. The drop of NaOH added after the equivalence point is reached turns the clear solution pink. The color change signals the end point of the titration which usually coincides very closely with the equivalence point.

10. Knowing the total volume of NaOH added to the acid solution to bring about neutralization, a student/chemist can use stoichiometry calculations to determine the concentration of the acid. • See sample exercise 4.16 and practice exercises. • For example: A student is trying to determine the concentration of an NaOH solution. She neutralizes a 20.0-mL sample of the NaOH solution with with 45.7 mL of 0.500 M H2SO4. What is the molarity of the NaOH solution? • Write the balanced equation: H2SO4 + 2 NaOH Na2SO4 + 2 H2O • Calculate moles of H2SO4 used to neutralized the NaOH: • Use the mole ratio to calculate moles of NaOH in sample: • Calculate molarity of the NaOH: