Name: Petya Pisan______
Purpose: To determine the concentration of solution of sodium hydroxide by acid-base titration and analyse the shape of a titration curve.
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Pre-Lab: / 11 I /8 K/U / 5 C
1) What colour is the indicator phenolphthalein in an acidic solution and in a basic solution? (2
K/U)
Phenolphthalein turns colourless in acidic solutions and pink in basic solutions.
2) Why does the clear colour, which forms at the point where the hydrochloric acid comes into contact with the solution containing phenolpthalein and base in the receiving flask, disappear more slowly near the endpoint? (2I)
As the equivalence point is being reached the number of moles of acid start to neutralize the number of moles of base and therefore the clear colour disappears more slowly and the pink colour starts to form because you are approaching the equivalence point.
3) What ions are taking part in the neutralization reaction? (2 I)
“In the reaction of an acid with a base in aqueous solution, the hydrogen ions of the acid react with the hydroxide ions of the base to give water. The second product is a salt, which is composed of the positive metal ion from the base and the negative ion from the acid.” http://www.chemistryexplained.com/A-Ar/Acid-Base-
Chemistry.html
4) In terms of the ions present, what has happened at the endpoint of the titration? (1 I)
The ions will have cancelled themselves out and a neutral solution will result.
5) Why is it a good idea to carry out titrations in triplicate? (1 I)
To rule out false readings and ensure the best possible result. To be able to take an average of the three results increases the statistical precision.
6) Would the addition of several millilitres of distilled water to the Erlenmeyer receiving flask during a titration affect the results of the titration? Explain your answer. (3 I)
Adding several millilitres of distilled water to the Erlenmeyer flask before titrating should not affect the results of the titration as there will still be the same quantity of sodium hydroxide solution but in a more dilute concentration. What is reacting is the number of moles (particles, molecules, atoms) of acid (HCl) with the base (NaOH). When the number of moles of acid equal the number of moles of base, the equivalence point has been reached.

7) Explain what the shape of the titration curve (a plot of pH vs. volume of acid added) would look like for this experiment. Sketch the curve.(2 I)
8) If 27.31 mL of 0.2115 mol/L sodium hydroxide is able to neutralize 37.45 mL of hydrochloric acid, what is the concentration of the acid? (3 K/U) Please see scanned pages for answers.
9) What volume of 0.117 mol/L hydrochloric acid is needed to neutralize 28.67 mL of 0.137 mol/L potassium hydroxide? (3 K/U) Please see scanned pages for answers.
10) Draw a flow chart for this experiment and prepare your data table. (5 C) Please see scanned pages for answers.
Materials:
 250 mL beakers
 250 mL Erlenmeyer flask
 50 mL buret
 Standardized solution of hydrochloric acid
 Unknown solution of sodium hydroxide
 Phenolphthalein
Procedure:
1) Prepare a data table as shown. Record all of your experimental results in the table as soon as you obtain them.

2) Remember to clean ALL glassware before you use it.
3) Obtain about 120 mL of hydrochloric acid solution in a 250 mL beaker
4) To clean the buret - make sure it is closed and, using a funnel, pour about 10 mL of the hydrochloric acid solution into the buret. Place the buret in a buret holder on a retort stand and place a catch beaker under the spout of the buret and let the hydrochloric acid run through. Repeat this procedure twice more using new 10 mL samples of hydrochloric acid each time.
5) Refill the buret so that the meniscus of the hydrochloric acid solution is above the 0 mL mark and let the excess solution run out so that the meniscus eventually rests on the 0 mL mark. If there is a drop of solution hanging on the buret tip, remove it by touching the drop to the inside wall of the 250 mL
Erlenmeyer flask.
6) Obtain 120 mL of sodium hydroxide in another (clean!) 250 mL beaker.
7) Pipette 25 mL of the sodium hydroxide into the Erlenmeyer flask.
8) Place 3 drops of phenolphthalein into the Erlenmeyer flask that contains the base.
9) Place the Erlenmeyer flask under the buret.
10) Slowly add hydrochloric acid from the buret, one mL at a time. Stop after every 1 mL addition to swirl the solution in order to make sure that the acid is properly mixed with the base. You will start to see the pink colour change. When you get the sense that you are getting close to the endpoint add the acid
drop by drop and swirl continuously so that you don’t miss it. The endpoint has been reached (or passed) when the indicator changes colour permanently. The endpoint will occur suddenly and you may overshoot it the first time you do the titration. If you think you passed the endpoint by several drops redo the titration before recording the volume of acid added.
11) Repeat the titration (steps 7 – 10) twice more. Do not refill the buret unless you have run out of acid.
Record the initial reading of the buret and add acid from that point. Record all of your data. Calculate the average volume of acid added.
Observations:
Titrations with indicator:
Measurements
Concentration of HCl (mol/L)
Trial #1
0.1M
25.5 mL
Trial #2
0.1M
25.0 mL
Trial #3
0.1M
25.3 mL Volume of NaOH (mL)
Initial reading of acid on buret (mL)
Final reading of acid on buret (mL)
Total volume of HCl added (mL)
Average Volume of HCl added (mL)
Calculations: (show all calculations and formulae clearly)
1.
Determine the number of moles of hydrochloric acid used. (2 A)
2.
Determine the number of moles of sodium hydroxide used. (1 A)
3.
Determine the concentration of the unknown sodium hydroxide solution. (2 A)
Marking Scheme for Lab:
10 marks Communication based on communication rubric and proper lab format
15 marks Application (3 data table, 5 analysis , 2 error analysis, 2 conclusion, 3 lab performance)
TOTAL /25