Question

I need to write a lab plan based on preparing a standard solution-- Standardization of NaOH (aq)

Experiment 8: Preparing a Standard Solution—Standardization of NaOH(aq) Learning Objectives » to understand the meaning of the term primary standard » to learn how to determine the precise concentration of a solution » to learn how to properly prepare, use and read a buret » to practice titration techniques and calculations » to learn how to weighing by difference Read and complete the Prelab Assignment for Experiment 8 (Chemistry 121 Lab Moodle site). Note that these assignments are always due the Sunday the week of lab, by 5 PM. Safety: 3.0 M sodium hydroxide is hazardous and will attack skin and clothing rapidly. Don't wear expensive clothes to lab. Flood any spills with water and ask your instructor or assistant for help. Wear goggles! If solutions are splashed on your face, call for help and use the eyewash. Materials 3 M NaOH, solid KHP (potassium hydrogen phthalate) dried, in weighing bottles, phenolphthalein indicator solution, one buret, four 250-mL Erlenmeyer flasks. Disposal The solutions of KHP and dilute NaOH should be neutralized (5 < pH < 9) and only then may be poured down the drain with lots of water. This is a skill-building experiment, where you will have to work carefully and use equipment skillfully in order to achieve good results. Learning good titration techniques will be helpful in subsequent labs, not only as specific skills, but they illustrate the excellent results that can be achieved if sound technique is employed. Students can work individually or in pairs on this experiment. Remember, it's up to you to make sure that you take part in all lab activities!

Answer 1

The problem with NaOH is that it is hygroscopic, it absorbs water. So when you weigh out a mass of NaOH to prepare a solution some of the mass is actually absorbed water, thus you can never be sure of the exact concentration of NaOH in your solution.

So when using NaOH it is standard practice to standardize the solution before using it. This means titrating the NaOH solution with an acid that you are absolutely sure of the number of moles of. KHP (potassium hydrogen phthalate) is one of the more common acids that you use.

KHP is monoprotic. 1mole KHP neutralises 1 mol NaOH

KHC8H4O4 (aq) + NaOH (aq) → NaKC8H4O4 (aq)+ H2O(l)

Weigh accurately a known mass of KHP (usually to 4 places) into a flask.

Generally select a mass that you will need a decent volume of NaOH to titrate with, say 15-25 ml.
you know that your NaOH is approx 3 M, so 20 ml will contain approx 0.6 mole of NaOH.
So a good mass of KHP will be around 0.6 mol x 204.22 g/mol = 123 g

dissolve it in some distilled water. Then titrate it with your ~ 3M M NaOH.

Since you know the exact moles of KHP, you will determine the exact moles of NaOH in the volume used to titrate. This allows you to work out the actual concentration of the NaOH that you prepared. Usually you would do 2-3 titrations and take an average...

take the NaOH 15 mL solution and add little indicator (solution turns to pink). Then titrate it against KHP solution and note down the volume.

indicator of equilibrium point: solution colour (pink) changes to colourless.

At the equivalence point : # mol KHP x 1 mol NaOH /(1 mol KHP) = # mol NaOH

Molarity of NaOH = # mol NaOH/titration volume in L

Then, knowing the actual concentration of the NaOH solution you can use it to titrate the vinegar and determine the % acid