COPPERBELT UNIVERSITY SCHOOL OF MATHEMATICS AND NATURAL SCIENCES 1st Year Chemistry Experiment 3 – 2022 PREPARATION AND STANDARDIZATION OF SOLUTIONS – SODIUM HYDROXIDE SOLUTION STANDARDIZED BY TITRATION WITH KHP OBJECTIVE The objective of this experiment is to learn: i) How to prepare a solution (sodium hydroxide) of a certain concentration ii) How to standardize the prepared solution using a primary standard (potassium hydrogen phthalate – KHP) as primary standard and iii) How to conduct a titration of solutions to determine the endpoint of a reaction. DISCUSSION Preparation of standard solutions in chemistry laboratory work is an essential technique that enables analysts to produce accurate and precise data that ensures reliable analytical results. The need to develop the skill early guarantees confidence in a student’s future handling of related laboratory analyses. In order to determine the concentration of a solution, there must be something to compare it against (benchmark). This benchmark is referred to as a standard. In this experiment, an acid-base reaction is to be carried out to demonstrate the concept. Sodium hydroxide (a base) is typically used as a standard to determine the concentration of acids. When an acid and a base react, they form salt and water. In this case, KHP is the acid being reacted with the base (NaOH). KHP is the standard, KHC8H4O4(aq) + NaOH(aq) → KNaC8H4O4(aq) + H2O(l) Acid Base → Salt + Water In a titration, the volume of a solution added to a reaction is measured using a burette. In this case, the acid and base are reacted together until one of the two is completely reacted or neutralised. That neutral point is called the endpoint. If any additional acid or base is added, the solution will then become acidic or basic depending on which was added in excess. To visually determine the endpoint, an indicator is added to the reaction vessel. An indicator is a chemical that changes colour at a particular pH. When just a tiny excess of the acid or base is added beyond the completion of the reaction, the indicator changes colour. The amount added from the burette at this point is called the endpoint volume. 1 In this experiment, a sodium hydroxide solution (NaOH) will be prepared. In order to standardize the sodium hydroxide solution, there must be a substance to standardize the prepared solution against. This substance will be potassium hydrogen phthalate (KHP). KHP is an organic acid that is solid and thus, easily weighed. Using basic stoichiometry, the moles of NaOH in the solution can be determined from the moles of KHP added to the reaction, thereby enabling us to calculate the concentration of the NaOH. EQUIPMENT AND CHEMICALS Sodium hydroxide Phenolphthalein indicator KHP 100 ml volumetric flask 25 ml burette 100 ml Erlenmeyer flask 50 ml graduated cylinder 100 ml beaker PROCEDURE I. Preparation of a NaOH Solution 1. Weigh, as accurately as possible, 0.5g of NaOH into a clean 100 ml beaker. (Caution: NaOH is caustic and can cause burns. Avoid contact with skin) 2. Add 50 ml of distilled water to dissolve the pellets by carefully swirling the beaker. 3. Carefully transfer the solution into a clean 100ml volumetric flask. Use a wash bottle to wash off the contents of the beaker with distilled water and transfer the washings into the volumetric flask. 4. Make up the volume of the solution to the mark with the help of a wash bottle. II. Preparation of burette 5. Clean a 25 ml burette and rinse with distilled water. A clean burette will have no droplets clinging to the inside of the glass. 6. Rinse the burette with two 5 ml portions of your NaOH solution. Make sure you drain the NaOH solution through the tip of the burette and dispose of the used NaOH solution. 7. Fill the burette with the NaOH solution. Make sure that the tip is also filled with no air bubble(s). 8. Slowly drain the NaOH out of the burette until the burette reads approximately 0.0 ml. . III. preparation of KHP Standard 9. Weigh to the nearest milligram 0.1 grams of dried KHP (potassium hydrogen phthalate) in a clean 100 ml Erlenmeyer flask. 10. Add 20 ml of distilled water to the flask. 11. Add three drops of phenolphthalein indicator. IV. Standardization of NaOH 12. Secure a white background underneath the flask with the KHP solution. 13. Slowly add, with constant swirling, the NaOH drop-wise to the KHP solution. 14. Continue adding NaOH drop wise (with constant swirling) to the KHP solution until the KHP solution turns a faint shade of pink that persists. This is called the endpoint. 15 Repeat steps 9 – 14 four additional times, recording you data on the data sheet. 2 DATA ANALYSIS 1. Calculate the concentration of the NaOH prepared in stage I of the procedure. 2. Calculate the number of moles of KHP used in each titration. Enter your answers on the data sheet. 3. Calculate the molarity of each NaOH solution in each titration and fill out your data sheet. 4. Calculate the average molarity in moles/litre of the NaOH solution. 5. Using the values calculated in (1) and (4), determine the percent error of your experiment. POST-LAB QUESTIONS 1. In every titration, we have a titrant and an analyte. Explain the two terms. 2. In this experiment, identify the titrant and the analyte. 3. What do you understand by the term equivalence point? 4. Explain the accuracy of your endpoint with respect to the equivalence point. 3 STANDARDIZATION OF NaOH DATA SHEET – EXPERIMENT 3 Preparation of the primary KHP solution Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Mass of KHP Formula mass of KHP (204.2 g/mole) 204.2 Moles of KHP (mass of KHP / formula mass KHP) Titration of KHP Final burette reading in ml Initial burette reading in ml Volume of NaOH added (final – initial) in ml Volume of NaOH added in litres (ml / 1000) Molarity of NaOH (moles KHP / litre NaOH) Average Molarity of NaOH 4 204.2 204.2 204.2 204.2
