STANDARDIZATION OF A SODIUM HYDROXIDE SOLUTION An acid - base or neutralization reaction can be represented as. acid + base > salt + water

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1 49 Standardization of a Sodium Hydroxide Solution STANDARDIZATION OF A SODIUM HYDROXIDE SOLUTION An acid - base or neutralization reaction can be represented as acid + base > salt + water The reaction between a generic acid (H + ) and a generic base (OH - ) can also be written as: H + + OH > H 2 O or H 3 O + + OH > 2H 2 O In this experiment, you will use this reaction to accurately determine the concentration of a sodium hydroxide (NaOH) solution that you have prepared. The process of determining the concentration of a solution is called standardization. To accomplish this, you must accurately measure, using a buret, the volume of one reagent that is required to exactly react with another reagent; this is termed titration. An indicator solution can be used to determine when an acid has exactly neutralized a base or vice versa. Acid-base indicators work because they can exist in two different forms, each of which is a different colour. At low ph values, the indicator is in its acidic form. At high ph values, the indicator loses a proton (H + ) and is converted into its basic form. The ph at which this occurs depends on the indicator. HIn > H + + In - (colour A) (colour B) The indicator you will use is phenolphthalein. Its behaviour is slightly more complex in that it loses a molecule of water at the same time as it loses the proton. It has a colourless acidic form and a bright pink basic form in equilibrium with each other as follows: Phenolphthalein changes colour over the ph range of As soon as you see any persistent faint pink colour, you should stop your titration. This is the end point, the point at which the indicator changes colour. You want the end point to be as close as possible to the equivalence point, the point at which you have added exactly equivalent amounts of acid and base. To get consistent results, it is therefore essential to titrate to the same shade of faint pink every time.

2 Standardization of a Sodium Hydroxide Solution 50 In this experiment, you will prepare a solution of NaOH which you then must standardize due to the fact that NaOH is unavailable in pure form. Commercial grade NaOH is always contaminated by moisture and up to 2% sodium carbonate (Na2CO3). After you have prepared a solution of approximately the desired concentration, you will standardize it by using it to titrate a measured mass of a primary standard acid. A solution of the primary standard acid may be prepared by direct measurement of the mass of the acid and the volume of the solution prepared. There are several requirements that must be met in order for an acid to qualify as a primary standard acid: 1. It must be obtainable in pure form. 2. It must be stable in pure form, as well as in solution. 3. It must be easily dried and nonhygroscopic (does not absorb water). 4. It must be soluble in the solvent you plan to use. 5. It must be capable of entering into a stoichiometric reaction with the base to be standardized. 6. It should have an adequately high molecular weight so that the quantity weighed out is large enough to prevent any significant weighing error. The primary standard acid you will use for standardization of your NaOH solution is potassium hydrogen phthalate (KHC 8 H 4 O 4 ). A commonly used abbreviation for potassium hydrogen phthalate is KHP. Solutions consist of one substance (the solute, or dissolved substance) dissolved in another substance (the solvent, or dissolving substance). The most common solvent for laboratory solutions is liquid water. For example, your solution of NaOH will be prepared by dissolving solid NaOH pellets (the solute) in water (the solvent). A commonly used unit of solution concentration is molarity (M). Molarity is defined as the number of moles of solute per liter of solution prepared: molarity moles of solute volume of solution (in L) or M n V Recall that you can calculate the moles of solute from its mass and molar mass: moles mass (in g) or n molar mass (in g/mol) Note that molar mass goes by many different abbreviations; your textbook probably refers to it as M. To avoid confusion with molarity (also M), we will use MW for now. m MW

3 51 Standardization of a Sodium Hydroxide Solution Once you have found the end point of your titration (and therefore approximated the equivalence point), you are then able to determine the concentration of the solution whose concentration was unknown. The stoichiometry of the reaction must be known. For example, consider the following generic acid-base reactions: HA + NaOH > NaA + H 2 O (1) H 2 A + 2 NaOH > Na 2 A + 2 H 2 O (2) In reaction (1), one mole of acid HA reacts with one mole of base (NaOH). Therefore, an equation may be written: moles of NaOH moles of acid Which allows calculation of the molarity of the sodium hydroxide solution: M NaOH moles of NaOH volume of NaOH (in L) In reaction (2), one mole of acid H 2 A reacts with two moles of base (NaOH). Therefore, the equation would be written 2 moles of NaOH moles of NaOH moles of acid! 1mole of acid Again, this would allow calculation of the molarity of the sodium hydroxide solution: M NaOH moles of NaOH volume of NaOH (in L) KHP reacts with NaOH with a 1 : 1 stoichiometry as in reaction (1) above.

4 Standardization of a Sodium Hydroxide Solution 52 Experimental Procedure This is an individual lab. In preparation for this procedure, read Appendix B for directions on the proper use of a balance and how to weigh by difference, and Appendix C for directions on the proper use of a buret in a titration. SAFETY NOTE: Sodium hydroxide is a strong base and can cause burns if it is left on the skin for too long. If your hands feel slippery or soapy at any point during the lab, wash them well to remove the sodium hydroxide. Weigh out, on a top-loading balance, the quantity of NaOH pellets required to prepare one litre of approximately 0.1 M NaOH solution. Immediately after weighing, dissolve the pellets in about 450 ml of distilled water in a 600 ml beaker. This is an exothermic process, so the water will heat up quite a bit. This will help the pellets dissolved, but you should cool the beaker if it gets too hot to touch. Pour the solution into a clean brown bottle and fill to the neck with distilled water. Cap the bottle and shake well. Label the bottle with the solution name, your name, lab section number, and the semester. Save this solution. Keep your NaOH solution capped as much as possible; otherwise, CO2 in the air will react with the NaOH solution and change its concentration. Weighing by difference using an analytical balance, accurately measure out three samples of KHP in the range of g into separate Erlenmeyer flasks. To each portion of KHP, add approximately 50 ml of steaming distilled water, swirl until the solid has dissolved, and cool to room temperature. Add 2 4 drops of 0.1% phenolphthalein indicator solution to each flask. Titrate the KHP acid sample in one of the flasks with the prepared NaOH solution until a stable faint pink end point is reached. The end point should last for at least 15 seconds (absorption of CO 2 from the air will cause the colour to eventually fade). Proceed to titrate the two remaining flasks. It is a good idea to titrate the smallest mass of KHP first. Calculate the molarity of your NaOH solution for each trial run, then calculate the average molarity of the solution. You may use this value for further calculations in the next experiment, if you feel all runs were done well.

5 53 Standardization of a Sodium Hydroxide Solution Standardization of a Sodium Hydroxide Solution DATA SHEET: Individual Report Name: Lab Section: Standardization of a NaOH Solution Trial 1 Trial 2 Trial 3 Trial 4 Mass KHP + paper Mass paper Mass KHP Final volume reading (Vf) Initial volume reading (Vi) Volume ~0.1 M NaOH used Observations Instructor's signature: Results: Trial # 1: Trial # 2: Trial # 3: Trial # 4: ( If needed ) Average molarity of NaOH:

6 Standardization of a Sodium Hydroxide Solution 54 Standardization of a NaOH Solution: Pre-lab Questions 1. Give the formula to define the term molarity (M). Include units. 2. Define the term standardization of a solution. How is this accomplished? 3. Write the true chemical formula for potassium hydrogen phthalate (KHP). 4. Calculate the molar mass of potassium hydrogen phthalate (KHP). 5. List four requirements an acid must meet before it can be referred to as a primary standard acid. 6. What is the purpose for the addition of an indicator solution to the NaOH-KHP reaction? 7. In this experimental procedure, which indicator solution will be used? 8. Balance the following neutralization reaction: H 2 SO 4 + NaOH > Na 2 SO 4 + H 2 O 9. How many moles of NaOH are required to exactly neutralize moles of KHC 8 H 4 O 4? 10. Write the net ionic equation for a strong acid-strong base neutralization reaction. 11. Calculate the molarity of a NaOH solution when ml of the NaOH was required to neutralize a solution containing g of KHP. 12. What colour is each form of phenolphthalein? 13. Why must a solution of NaOH be standardized? 14. Define the terms equivalence point and end point. 15. Define the term titration. 16. What is the expected colour change at the endpoint in the NaOH-KHP reaction?

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