ACIDS AND BASES

Acid Base Titration A very accurate method to measure concentration. Acid + Base  Salt + Water H + + OH -  H 2 O Moles H + = Moles OH -

Buret Solution with Indicator

Acid-Base Titrations The amount of acid or base in a solution is determined by carrying out a neutralization reaction; an appropriate acid-base indicator (changes color in specific pH range) must be used to show when the neutralization is complete. This process is called a TITRATION: the addition of a known amount of solution to determine the volume or concentration of another solution.

3 steps… Add a measured amount of an acid of unknown concentration to a flask. Add an appropriate indicator to the flask (i.e. phenolphthalein) Add measured amounts of a base of known concentration using a buret. Continue until the indicator shows that neutralization has occurred. This is called the end point of the titration.

Example: A 25-mL solution of H 2 SO 4 is neutralized by 18 mL of 1.0 M NaOH using phenolphthalein as an indicator. What is the concentraion of the H 2 SO 4 solution? Equation: 2NaOH + H 2 SO 4  2H 2 O + Na 2 SO 4 How many mol of NaOH are needed for neutralization?

Example: A 25-mL solution of H 2 SO 4 is neutralized by 18 mL of 1.0 M NaOH using phenolphthalein as an indicator. What is the concentraion of the H 2 SO 4 solution? Equation: 2NaOH + H 2 SO 4  2H 2 O + Na 2 SO 4 How many moles of H 2 SO 4 were neutralized?

Example: A 25-mL solution of H 2 SO 4 is neutralized by 18 mL of 1.0 M NaOH using phenolphthalein as an indicator. What is the concentraion of the H 2 SO 4 solution? Equation: 2NaOH + H 2 SO 4  2H 2 O + Na 2 SO 4 Calculate the concentration of the acid:

Titration Curve: a graph showing how the pH changes as a function of the amount of added titrant in a titration. Data for the graph is obtained by titrating a solution an measuring the pH after every drop of added titrant.

Titration Curve: a graph showing how the pH changes as a function of the amount of added titrant in a titration. Equivalence point: the point on the curve where the moles of acid equal the moles of base; the midpoint of the steepest part of the curve (the most abrupt change in pH) is a good approximation of the equivalence point.

Titration Curve: a graph showing how the pH changes as a function of the amount of added titrant in a titration. Knowledge of the equivalence point can then be used to choose a suitable indicator for a given titration; the indicator must change color at a pH that corresponds to the equivalence point.

Calculations of Titrations

1) The Mole Method of Molarity: Calculate the molarity of a sulfuric acid solution if 23.2 mL of it reacts with g of Na 2 CO 3. H 2 SO 4 + Na 2 CO 3  Na 2 SO 4 + CO 2 + H 2 O

2) Normality: The normality (N) of a solution is defined as the (M) x (total positive oxidation)

Examples: HCl  H + + Cl - H 2 SO 4  2H + + SO 4 2- NaOH  Na + + OH - Ba(OH) 2  Ba OH - + ox = 1 + ox = 2 + ox = 1 + ox = 2

Example: Calculate the molarity and normality of a solution that contains 34.2 g of Ba(OH) 2 in 8.00 L of solution.

In titration problems, you can use this equation:

Example: 30.0 mL of N HNO 3 required 22.5 mL of Ca(OH) 2 for neutralization. Calculate the normality and molarity of the Ca(OH) 2 solution.