5 Sample ProblemIn this sample titration, we are trying to determine the concentration of mL of HCl. In the titration we will be neutralizing the HCl with M NaOH.
6 Step 1: The NaOH, the titrant, is placed in the buret Step 1: The NaOH, the titrant, is placed in the buret. The titrant is the solution of known concentration that is added from the buret.
7 Step 2: The HCl is placed in the Erlenmeyer flask along with approximately mL of distilled water and 2-3 drops of phenolphthalein indicator. Since the solution in the flask is acidic, phenolphthalein is colourless.
8 Step 3: NaOH is added to the HCl in the flask Step 3: NaOH is added to the HCl in the flask. When the NaOH comes in contact with the solution in the flask, it turns pink and then the pink colour quickly disappears. This is because the OH- from the NaOH interact with the phenolphthalein to change the phenolphthalein from colourless to pink.
9 The solution becomes clear again as the hydronium ions from the hydrochloric acid neutralize the added hydroxide ions. As more NaOH is added, it takes longer for the pink colour to disappear.As it starts taking longer for the pink colour to disappear, the sodium hydroxide is added a drop at a time.
11 The equivalence point of the titration is reached when equal numbers of moles of hydronium and hydroxide ions have been reacted.When this happens in this titration, the pH of the solution in the flask is 7.0 and the phenolphthalein indicator is colourless.This would be a good time to stop, however the indicator is still colourless, so must keep going.
13 Step 4: Add as little excess NaOH as possible Step 4: Add as little excess NaOH as possible. We want to add a single drop of NaOH to the colourless solution in the flask and have the solution in the flask turn pink and stay pink while the contents of the flask are swirled.This permanent colour change in the indicator is known as the endpoint of the titration and the titration is over.
14 Titration CurveZumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 527
15 Solve the problem 1st write the equation for the reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)2nd solve for the amount of moles of the titrant used.NaOH mol = mol/L x L= 3.85 x 10-3 mol NaOHFound in titration experiment3rd using stoichiometry, solve for the concentration of HCl , knowing it is a 1:1 mole ratio3.85 x 10-3 mol= ML
16 SAMPLE PROBLEM 2In an acid-base titration, mL of M nitric acid, HNO3, were completely neutralized by mL of aluminium hydroxide, Al(OH)3. Calculate the concentration of the aluminium hydroxide.
17 SAMPLE ANSWER 2 The balanced equation for the reaction is: 3HNO3(aq) + Al(OH)3(aq) → Al(NO3)3(aq) + 3H2O(l)The number of moles of nitric acid used is:y mol = mol/L x L = 3.14 x 10-3 mol HNO3From the stoichiometry of the reaction, the number of moles of aluminium hydroxide reacted is:3.14 x 10-3 mol HNO3 x 1 mol Al(OH)3 = 1.05 x 10-3 mol3 mol HNO3Therefore, the concentration of the aluminium hydroxide is:1.05 x 10-3 mol Al(OH)3 = ML