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Lab Title: Titration The purpose of this lab is to: PURPOSE Determine the molarity of 2 different sodium hydroxide (NaOH) solutions of unknown concentration by titrating with a standard 0.1 M HCl solution Calculate percent error of results MATERIALS 2 burettes 2 different NaOH 2 Erlenmeyer flasks (250 mL) solutions of unknown 0.1 M HCl solution concentration Phenolphthalein indicator Gloves (optional) You experienced how to determine the approximate pH of a PRE-LAB solution by using pH indicators in the Household Chemicals lab. DISCUSSION What if you need to know the exact pH, or hydrogen ion concentration, of a solution? As an example, environmental scientists, studying the effects of acid rain, might need to quantify the exact hydrogen ion concentration in Adirondack lake waters. There are several ways to quantify hydrogen or hydroxide ion concentration. Hand-held electronic pH meters are often used. An alternative, and much more colorful method, is called titration. Page 1 ARHS Chemistry Dept. When an acid and a base combine with each other, a neutralization reaction occurs. The general form of the neutralization reaction is: acid + base salt + water Titration makes use of the neutralization reaction to quantify the concentration of an unknown acid or base. The specific neutralization reaction that you will conduct today is: HCl + NaOH NaCl + H2O The figure on the left side of page 1 of your lab shows how pH changes during the titration of a strong acid (such as HCl) with a strong base (such as NaOH). The starting pH is low. As base is added, the pH rises. The rise is pH is gradual at first. However, when nearly all of the H+ ions from the acid are neutralized by the base, the pH shoots up dramatically with the addition of a very small volume of NaOH. This abrupt increase in pH occurs at the equivalence point of the titration. By choosing a pH indicator that changes color near the equivalence point, we get a visual cue at that point in the reaction. The equivalence point for a strong acid (one that ionizes completely) and strong base is pH 7.0. Weak acid/base combinations have equivalence points near, but not equal to, pH 7.0. At the equivalence point, we know that the total number of OH- ions is equal to the total number of H+ ions. Therefore, we can apply the titration equation, shown below and in Reference Table T, to figure out the concentration (aka molarity) of our unknown solution. Titration Equation: MaVa = MbVb Ma = molarity of acid M b = molarity of base Va = volume of acid Vb = volume of base Page 2 ARHS Chemistry Dept. CHECK FOR UNDERSTANDING: 1. 400 ml of a solution contains 2 moles of H+ ions. What is the molarity of the solution? Show calculation with units: _________________________ 2. Identify the products of this neutralization reaction: HNO3 + KOH ________ + ________ 3. 50 ml of 0.2 M HCl is used to titrate 22 mL of an NaOH solution. Show a correct numerical set up (including units) of the titration equation for this situation: 4. Solve the equation above for the molarity of the base. Show all work and a final result ______________M NaOH PROCEDURE 1. Work in teams of 2-3. Each partner does at least one titration. 2. Deliver approximately 10 ml of acid into an Erlenmeyer flask. Record initial & final burette readings on data sheet to the nearest 0.01 ml and calculate net volume of acid to the nearest 0.01 ml. 3. Add 1-2 drops of phenolphthalein to flask 4. Record initial base burette reading. SLOWLY add base to acid while swirling until a light pink color is seen. Record final burette reading and calculate net volume of base added to the nearest 0.01 ml. 5. Use the titration equation to calculate the experimental molarity of the base. 6. Ask your teacher for the actual molarity of your base. Calculate percent error. 7. Repeat for a total of two bases – two trials per base. Caution: Be sure to wear safety goggles to protect your eyes from acid/base.. Page 3 ARHS Chemistry Dept. Data Sheet 1 Base #_______ Determine the molarity of the NaOH Trial 1 of 2 Show all calculations here 0.10 M HCl NaOH Initial Volume (ml): +/- 0.01ml Final Volume (ml): +/- 0.01ml Volume Delivered (ml) +/- 0.01ml Base #_______ Determine the molarity of the NaOH Trial 2 of 2 Show all calculations here 0.10 M HCl NaOH Initial Volume (ml): +/- 0.01ml Final Volume (ml): +/- 0.01ml Volume Delivered (ml) +/- 0.01ml Page 4 ARHS Chemistry Dept. Data Sheet 2 Base #_______ Determine the molarity of the NaOH Trial 1 of 2 Show all calculations here 0.10 M HCl NaOH Initial Volume (ml): +/- 0.01ml Final Volume (ml): +/- 0.01ml Volume Delivered (ml) +/- 0.01ml Base #_______ Determine the molarity of the NaOH Trial 2 of 2 Show all calculations here 0.10 M HCl NaOH Initial Volume (ml): +/- 0.01ml Final Volume (ml): +/- 0.01ml Volume Delivered (ml) +/- 0.01ml Page 5 ARHS Chemistry Dept. Data Summary & Analysis Base #:______ Base #:______ Trial 1 Molarity: Trial 1 Molarity: Trial 2 Molarity: Trial 2 Molarity: Average Average Molarity: Molarity: Actual Molarity Actual Molarity (from teacher): (from teacher): % Error: % Error: (show calculation (show calculation below) below) Calculation: Calculation: REAL LIFE CONNECTION Medical and Pharmaceutical Analyses Ever have blood drawn for testing after a doctor’s visit?? Chances are, a titration process was used to analyze your blood. Many common blood tests, such as glucose monitoring in patients with diabetes, are done colorimetrically using titration methods. Urinalysis, such as pregnancy testing, is frequently based on titration. Doctors & pharmacists use titration to determine the correct proportion of different medicines in an intravenous drip. Most modern blood labs use automated titration equipment, but the concepts behind the equipment are exactly those you did in this lab! Page 6 ARHS Chemistry Dept. CONCLUSION Type your conclusion in CEI format on a separate sheet of paper. You must use the following vocabulary terms in a way that demonstrates their meaning: Neutralization Endpoint Titration Molarity Hydroxide ion Hydrogen ion alkaline acid Be sure to UNDERLINE OR HIGHLIGHT each of the vocabulary words. Page 7 ARHS Chemistry Dept.
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