Download Experiment: Titration and more Lab Reports Chemistry in PDF only on Docsity!
Experiment: Titration
INTRODUCTION
In this experiment you will be determining the volume of sodium hydroxide solution of known concentration required to neutralize a known mass of an unknown acid in solution. The technique used will be titration.
Terms you will need to be familiar with in order to understand a discussion of titration are:
titration : a technique used to measure the volume of a solution of known concentration that is required to react with a measured amount (mass or volume) of an unknown substance in solution.
buret : an instrument used to measure volume; a graduated glass tube about 40 cm long with a stopcock on one end. The volume measurement is made by reading the fluid level in the buret before and after the titrant , the fluid in the buret, is dispensed through the stopcock.
standard solution : a solution of known concentration.
unknown : a substance or a mixture about which something is not known.
indicator : a substance which is added to the reaction system in small amounts; it indicates that the reaction is complete (has reached the end point) by changing color.
end point : the stage in the titration at which the indicator color change is observed, indicating that the reaction is complete.
EXPERIMENT SUMMARY:
Your unknown solid is a monoprotic acid. Let’s call it HX. You will dissolve it in water, add one drop of indicator and then add just enough NaOH solution to neutralize the acid:
HX (aq) + NaOH (aq) H 2 O (l) + NaX (aq)
The indicator, phenolphthalein, is colorless in acidic solution but turns pink when there is an excess of base present, therefore you will know that the reaction is complete when you see the first faint hint of pink color. Because the pink color occurs when there is an excess of base present, the lighter the pink color is at the end point, the better. Also, the pink should last for 30 seconds after swirling the solution. After that time it may fade due to the presence of carbonic
acid, H 2 CO 3 , in the solution. This is formed when CO 2 in the air reacts with water:
CO 2 (g) + H 2 O (l) H 2 CO 3 (aq)
The H 2 CO 3 reacts to neutralize the NaOH that was causing the pink color, and the pink color
goes away.
You will weigh your acid sample before you dissolve it in water and then you will measure the volume of NaOH solution required to neutralize the acid. Using these data you will calculate the mass of acid that would require 20.00 ml of standard base solution.
PROCEDURE
A. Obtain an unknown acid sample from your instructor. Clean three 125 ml Erlenmeyer flasks. Make sure the external walls of the Erlenmeyer flasks are completely dry. Number them 1, 2, and 3 using pencil on the white spot on the flask.
A reminder: Throughout the weighing process you should protect the Erlenmeyer flasks from your hands with a paper towel.
B. Weigh by difference three samples of unknown acid as follows:
- Take the items listed below with you to the weighing room:
- The Unknown acid sample.
- A clean 125 ml Erlenmeyer flask labeled #1. Make sure the external wall of the Erlenmeyer flask is completely dry.
- Your report sheet.
- A pen (not pencil).
- You need to weigh accurately about 0.15 g of the Unknown acid as follows: a) Turn the analytical balance ON.
b) After you see a display of 0.0000 g, place the Erlenmeyer flask labeled #1 on the pan of the analytical balance.
c) Close the balance windows and touch the TARE button. A “ 0.0000 g ” will be displayed regardless of the mass of the Erlenmeyer flask.
d)) Carefully pour about 0.15 g of the unknown acid (without spilling) into Erlenmeyer flask #1. Close the balance glass case windows. Record this reading to the one-tenth of a milligram on the report sheet in column 1.
e) When you are finished, remove the Erlenmeyer flask, close the balance windows, touch the TARE bar, and press the OFF button.
To dispense a second sample into another Erlenmeyer flask labeled #2 , repeat steps 1 and 2 given above. Record your data in column 2 on the report sheet.
To dispense a third sample into another Erlenmeyer flask labeled #3 , repeat steps 1 and 2 given above. Record your data in column 3 on the report sheet.
Return to your desk and place the samples out of the way while you prepare the buret for titration. Ask your instructor to check and initial your weighing data before you begin to titrate.
C. Check out a buret from the stockroom. Clean the tube by first rinsing it three times with tap water. (Do not contaminate the stopcock with tap water.) Then rinse the tube with distilled water, letting the water drain through the stopcock. While draining through the stopcock, be sure to turn the stopcock completely around once. Obtain a buret clamp from the drawer under the hood, attach it to a ring stand, and then attach your buret to the clamp.
D. Take a clean, dry 50 or 100 ml beaker to the reagent bench and get approximately 40 ml of standard NaOH solution. Rinse the buret with the NaOH solution by first adding about 5 ml of the solution to the buret, and then rolling the tube so that the solution comes into contact with the entire wall of the tube, and finally draining it through the stopcock. Repeat the rinsing process once more to be certain all rinse water from step C has been replaced by standard NaOH solution. -2-
REPORT SHEET NAME______________________
EXPERIMENT: TITRATION Last First Instructor’s initial______________
A) Acid Samples
Mass of Acid g g g Data Approval ________________ B) Standard Base 1 2 3 Initial Buret Reading ml verification_________
ml ml
Final Buret Reading ml ml ml Calculated Volume of Base (^) ml ml ml Data Approval ________________
CALCULATIONS:
A) Mass of acid sample that would require 20.00 ml of standard base: Sample #1 Sample #2 Sample # Setup:
Answer: g/20ml g/20ml g/20ml
B) Average mass of acid that would require 20.00 ml of standard base. Setup:
_______________ g/20ml Accuracy (from Instructor): _____________ % error
C) Precision: Sample #1 Sample #2 Sample # Deviation Setup:
Standard Deviation: Setup:
______________ g/20 ml Percent Deviation: Setup: _______________ %
QUESTIONS:
A. For each of the following questions, give complete setups and be sure your significant figures are correct. Include all units in your setups and answers.
- A student weighed an unknown acid sample by difference and found its mass to be 0.3064g. The uncertainty in each reading made on the balance he used was + 0.0003 g. Calculate the percent uncertainty in the mass of the sample. Setup:
Answer_______________%
- The unknown acid sample (refer to question #1) required 25.86 ml of standard base solution to neutralize it. The uncertainty in each buret reading was + 0.02 ml. Calculate the percent uncertainty in the volume of standard base. Setup:
Answer_______________%
- Calculate the grams of the student’s unknown acid sample that would neutralize 30.00 ml of the standard solution. Setup:
Answer_______________g
- Calculate the percent uncertainty to the answer to question #3. Setup:
Answer_______________%
- What would be the absolute uncertainty in the mass of the acid that is required to neutralize 30 ml base? Setup:
Answer_______________g
- On the basis of your answer to question #5, to how many significant figures should the answer to question #3(the grams acid to neutralize 30.00 ml of base) be reported? (No setup is required here.)
Answer_______________ B. What are two possible sources of error in a titration besides the unavoidable errors in reading a buret or balance?
