Acid-Base Neutralization Titration

Neutralization Mixing strong acids and bases produces a different kind of solution – a neutral solution. This solution is neither acidic or basic. What is produced? A salt and water: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) This is called a neutralization reaction. Can you think of another way to classify this reaction? Neutralization reactions are commonly used for salt production. After the salt is produced, the water is evaporated.

Common salts produced by neutralization ammonium sulfate (fertilizer), calcium sulfate dihydrate (gypsum, plasterboard), baking soda, sodium thiosulfate

Which reaction represents the process of neutralization? Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) HCl(aq) + KOH(aq) → KCl(aq) + H2O(l) Pb(NO3)2(aq) + CaCl2(aq) → Ca(NO3)2(aq)+ PbCl2(s) 2KClO3(s) → 2KCl(s) + 3O2(g)

Which balanced equation represents a neutralization reaction? H2SO4 + 2LiOH → Li2SO4 + 2H2O BaCl2 + Cu(NO3)2 → Ba(NO3)2 + CuCl2 2KClO3 → 2KCl + 3O2 Mg + NiCl2 → MgCl2 + Ni

Titration The neutralization of an acid by a base can be done very precisely using the technique of titration. In titration, a solution of known acidity, or pH, is added gradually to a solution of an unknown concentration of base. When the unknown solution is exactly neutralized, it is shown by the color change of an acid-base indicator or by the reading of a pH meter.

The net ionic equation corresponding to the neutralization of an acid by a base in aqueous solution can be written as follows: H+(aq) + OH-(aq) → H2O In the reaction, one mole of H+ is exactly neutralized by one mole of OH- ions.

Equal volumes of 0. 1 M NaOH and 0. 1 M HCl are thoroughly mixed Equal volumes of 0.1 M NaOH and 0.1 M HCl are thoroughly mixed. The resulting solution has a pH closest to  5  7  3  9

Titration Formula For monoprotic acids (HCl, HBr) and monohydroxy bases (NaOH, KOH), the number of moles of H+ or OH- ions is equal to the product of the molarity and volume: n=M x V. When the acid has neutralized the base (the equivalence point of the titration), the moles of acid equal the moles of base:

The origin of the titration formula At neutralization, moles of H+ = moles of OH-

Note that this simple formula does not work for acids with more than one ionizable hydrogen, such as H2SO4 or H3PO4, or for dihydroxy bases such as Ca(OH)2.

Tools of Titration In titration, the solutions are dispensed from burettes. The volume used of each solution is calculated by subtracting the volume read before the titration begins from the volume read after the titration is complete. The volume in a burette can be read automatically to 0.01 ml. This makes possible a very accurate calculation of the unknown solution, provided that the concentration of the known solution is accurately known. A solution whose concentration is known to high degree of accuracy is called a standard solution.

Burettes

Reading the Buret 0.50 M HCl unknown NaOH

Lab 27: Titration of an Unknown Base with a Known Solution of Acid In this experiment, you will use a standardized solution of HCl to titer a solution of NaOH of unknown concentration. Equipment 10 ml graduated cylinder goggles 250 Erlenmeyer flask Stirring rod 100 ml beaker 2 labeled burettes Glass funnels Burette stand Burette clamps funnels

Materials 0.10 M HCl unknown M NaOH phenolphthalein indicator

Safety Wear safety goggles at all times. NaOH is a very caustic material that can cause severe skin burns. Eye burns caused by sodium hydroxide are progressive: what at first appears to be a minor irritation can develop into a severe injury unless the chemical is completely flushed from the eye. If sodium hydroxide comes into contact with the eye, flush the eye with running water continuously for at least 15-20 minutes. Notify the teacher immediately.

Safety If NaOH is spilled on some other part of the body, flush the affected area with running water continuously for at least 10 minutes. Again, notify the teacher immediately if this happens. Hydrochloric acid is an irritant at the concentration used here. If you spill acid on yourself, immediately flush the affected area with water for 2-3 minutes and notify the teacher. If acid should get in your eyes, begin flushing your eyes with running water immediately and continue doing so for at least 20 minutes.

Procedure Measure 10.0 ml of HCl into a Erlenmeyer flask. Clean and dry a beaker, add NaOH and then fill the burette labeled NaOH. Take a reading and enter into data table. Make sure the tip of the burette is filled. Add 2-3 drops of phenolphthalein to the Erlenmeyer flask. Add NaOH from the burette to the Erlenmeyer, while swirling continuously. Titrate to a pale pink endpoint. A faint pink color will be present and will not disappear upon swirling. Try to get the drop that changes the solution from colorless to pale pink. Dark pink is too far – you must back titrate with acid. If this happens, be sure to record this change in your data table Read the NaOH burette and record in the data table. Repeat steps 1-8 for a second determination. Calculate the molarity of the NaOH using the formula given on the previous page.

Titration Data Table Initial Base Buret Data Trial 1 Trial 2 Trial 3 Final Base Buret Volume Base Used

Results (Show work for all calculations) Calculate the molarity of NaOH using the formula MAcidVAcid=MbaseVbase Obtain the accepted value for the concentration of your NaOH solution. Calculate your % error for each trial.

Sources of Error What are your possible sources of error in this experiment? How could you improve your results (minimize experimental error)?

Titration Problems (show work below) MacidVacid = MbaseVbase A student neutralized 16.4 ml of HCl by adding 12.7 ml of 0.620 M KOH. What was the molarity of the HCl acid? (Jan 04 exam)

MacidVacid = MbaseVbase In a titration experiment, a student uses a 1.4 M HBr(aq) solution and the indicator phenolphthalein to determine the concentration of a KOH solution. The data for trial 1 is recorded below. (Jan 05) Buret Readings HBr (aq) KOH (aq) Initial volume (ml) 7.50 11.00 Final volume (ml) 22.90 33.10 Volume used (ml) 15.40 22.10

Buret Readings HBr (aq) KOH (aq) Initial volume (ml) 7.50 11.00 Final volume (ml) 22.90 33.10 Volume used (ml) 15.40 22.10 show a correct numerical setup for calculating the molarity of the KOH(aq) solution in trial 1.

Why is it better to use several trials of a titration rather than one trial to determine the molarity of a solution of unknown concentration? In a second trial of this experiment, the molarity of KOH(aq) was determined to be 0.95 M. The actual molarity was 0.83 M. What is the percent error in the second trial? Show work below.