Chemistry Experiment: pH in Aqueous Solutions - Acetic Acid & Ammonia, Study notes of Chemistry

Download Chemistry Experiment: pH in Aqueous Solutions - Acetic Acid & Ammonia and more Study notes Chemistry in PDF only on Docsity! EXPERIMENT FOUR HYDROGEN ION CONCENTRATION AND pH OF AQUEOUS SOLUTIONS; DISSOCIATION CONSTANTS OF ACETIC ACID AND AMMONIA SOLUTIONS Water is a weak electrolyte. It dissociates slightly to give H + and OH - . H2O ↔ H + (aq) + OH - (aq) In pure water [H + ] = [OH - ] = 1.0 x 10 -7 M and thus the amount of dissociation is extremely small. The dissociation constant for water can be written, Kw = [H + ] [OH - ] / [H2O] = 1.0 x 10 -14 @ 25°C Chemists use activities in the formulation of K w instead of concentrations. By convention the activity of water is set equal to one and the activity of dilute solutions is set equal to the concentrations of the ion. Therefore our dissociation constant can be written as, Kw = [H + ] [OH - ] = 1.0 x 10 -14 We can use the dissociation constant of water to calculate the concentration of H + or of OH - in any aqueous solution if the concentration of the other ion is known. THE USE OF pH TO MEASURE ACIDITY It is generally easier to express [H + ] in terms of pH. We shall define pH = -log[H + ]. Thus the pH of pure water would be pH = -log[H + ] = -log[10 -7 ] = 7.0. Acidic solutions have a pH less than 7.0 and basic solutions have a pH larger than 7.0. THE LOWER THE pH THE HIGHER THE ACIDITY. DETERMINATION OF HYDROGEN ION CONCENTRATION AND pH In this section you will determine the [H + ] and pH of two unknown aqueous solutions. In the back of your Lab Manual you will find a table which will tell you the relationship between color and pH for several different indicators. While the data in this table are useful as a general guide, it should be realized that different individuals respond differently to the same color, and they may also use different words to describe a particular color. For the accurate determination of pH with an indicator, one must always make a direct comparison of the color of the unknown solution with the colors of solutions of known pH values. Furthermore, the solutions being compared should be as similar as possible in factors which may affect the color such as size of test tube, volume of solution, and number of drops of indicator. It is usually helpful to observe the colors by placing the test tubes side by side on a piece of white paper, and then looking down through the solutions from the top. EXPERIMENT: Obtain your unknowns from the instructional assistant. Test 2mL your first unknown with 2 drops of bromothymol blue to ascertain whether it is acidic or basic. Then, Docsity.com proceed with the appropriate indicators to determine the approximate pH. Record the name of each indicator. Use its color with your unknown and the pH range indicated into your data table. For these tests use 2 mL of your unknown solution and 2 drops of indicator. When you have decided the approximate pH, you will need to verify it. To do this you must prepare a solution of this pH and test it with the appropriate indicators. If your solution is in the 1 M to 10 -3 M H + concentration range (0 to 3 pH), prepare it from 6 M HCl. Solutions in the range from 10 -4 to lO -11 M H + (pH 4 to 11) are available as buffers from the instructional assistant. Solutions in the 10 -11 to 10 -14 M range of H + concentration range (pH 11 to 14) are prepared from 6 M NaOH. Use serial dilution to prepare solutions from HCl and NaOH. For these final critical comparisons use 5 mL of both the standard and the unknown (in different test tubes) and 3-4 drops of indicator. Compare the color of the standard solution to the color of your unknown with at least 2 and preferably 3 different indicators. If they are the same, then you know the pH and [H + ] of your unknown solution. AMMONIA SOLUTIONS Ammonia is a gas at room temperature that is very soluble in water. The resulting solution is basic because of the reaction, NH3(g) + H2O ↔ NH4 + (aq) + OH - (aq) Kb = [NH4 + ] [OH - ] / [NH3] EXPERIMENT : Prepare a 1 M solution of NH3 from the 6 M NH3 found in the lab. Determine the [OH - ] in the 1 M NH3 by using the indicators alizarin yellow and indigo carmine. Record the colors in your data table. Copy this table into your data section and complete it for 1 M NH3. This table and all similar tables must be copied into your data section. Substance H + OH - NH3 NH4 + Equilibrium Concentration Using these data calculate the dissociation constant Kb for NH3. Include this in your results. REACTIONS OF NH4 + AND OH - Predict what will happen when solutions of ammonium chloride (NH4Cl), and sodium hydroxide are mixed. EXPERIMENT: Prepare 12 mL of 1 M NaOH by adding 2 mL of 6 M NaOH to 10 mL of water. Mix thoroughly and then place 5 mL of the 1 M NaOH in each of two test tubes. Add several drops of indigo carmine to each test tube. Record the color in your data table. Slowly add 5 ml of 1 M NH4Cl to one test tube and record the color changes that occur. Does the OH - concentration increase or decrease as you add NH4Cl to the NaOH solution? Write an equation for the net reaction that occurs. Include this rxn and all similar reactions in your data section. Docsity.com