SCH3U
K
Acid/ Base Titration Lab
/4
TI
/8
Name: ______________________
MC
/6
C
/3
Introduction
It is difficult to make NaOH (aq) solutions with precisely known concentrations. This is due to 2 reasons:
1)
Sodium hydroxide is hygroscopic.
2)
Atmospheric CO2 (g) and dissolved CO2 (g) in the water reacts with NaOH and neutralizes some.
Purpose
To accurately determine the concentration of an unknown sample of sodium hydroxide by titrating it with a
known sample of hydrochloric acid.
Materials
Sodium hydroxide solution
Hydrochloric acid solution
Buret clamp
Retort Stand
Buret
Erlenmeyer Flask
(concentration unknown)
(concentration is 0.100 M)
Phenolphthalein Solution
Wash Bottle
100 mL beaker
250 mL waste beaker
Method
1.
2.
3.
4.
5.
6.
7.
8.
Set up the retort stand and clamp as demonstrated at the front of the classroom. Obtain a buret.
Rinse the buret using the following steps:
a)
Fill the 100 mL beaker with about 80 mL of NaOH (aq)
b)
Close the stopcock on the buret
c)
Transfer ~ 5 mL of NaOH (aq) to the buret. Open the stopcock to allow some of the base
to flow through the buret and into the waste beaker below. Pour the remainder of the
base out through the top of the buret while rotating to clean the buret walls.
d)
Repeat steps a-c to ensure a thorough cleaning.
Fill the buret with base using the following steps:
a)
Close the stopcock on the buret.
b)
Transfer the NaOH (aq) up to the fill line reading 0.00 mL. If you are above the line you
may allow some NaOH(aq) to run through the buret. Collect this NaOH(aq) in a clean dry
beaker for further use.
c)
Tap the buret gently to remove any air bubbles. Remove any excess NaOH(aq) at the
bottom of the buret by touching the bottom of the buret to your 100 mL beaker.
Obtain a clean Erlenmeyer flask (ideally 100 mL or 250 mL)
Bring the Erlenmeyer flask to the class buret of HCl (aq) at the front desk. Record the initial
reading on the buret in the table provided. Estimate the volume to the nearest 0.05 mL.
ALWAYS read the volume in the buret based on the bottom of the meniscus at eye level.
Allow about 15 mL of HCL (aq) to flow into the Erlenmeyer flask. Record the final volume in the
space provided.
Add 3 drops of phenolphthalein to your Erlenmeyer flask (otherwise no change in colour will
occur!)
Record the initial reading on your NaOH (aq) buret at your lab bench.
9.
Add the basic solution to the Erlenmeyer flask. Swirl often in order to mix the contents of your
flask. The end point of the titration is met when the first trace of pink colour remains even
after swirling the contents of the flask for 30 seconds. A piece of white paper beneath your flsk
may make this identification easier.
NOTE: If you overshoot the endpoint of the titration ( a dark pink ) you must take the following steps.
a)
add a few drops of stock acid until the solution becomes colourless
b)
add this amount to the original 15 mL you took (for calculation purposes)
c)
add base at your lab bench one drop at a time
d)
record the final volume of base that you took as a total
NOTE: When done properly a single drop of base will cause a permanent colour change - signifying
the end point of the titration
10.
11.
12.
Record the final volume of NaOH (aq) used to reach the endpoint.
Ensure that you have sufficient NaOH (aq) in the buret to perform the next titration. Repeat the
titration 3 times.
When finished
a)
discard any leftover solutions in the sink once you have ensured that your neighbours do
not require it
b)
rinse the beakers and Erlenmeyer flasks well with tap water and put the glassware away
c)
rinse the buret with distilled water, then with 5mL HCl (aq), then 3 more times with
distilled water
d)
place the buret in the buret container
Observations:
Trial #1
HCl (aq)
NaOH (aq)
Trial #2
HCl (aq)
NaOH (aq)
Trial #3
HCl (aq)
NaOH (aq)
Initial
Volume
Reading
(mL)
Final
Volume
Reading
(mL)
Analysis
1.
2.
3.
4.
5.
6.
7.
8.
Write a balanced molecular equation for the reaction occurring between hydrochloric acid and
sodium hydroxide. (K 4 marks)
Make a single set of calculations for the following calculations for trial #1 only: (TI 6 marks)
a)
the volume of HCl (aq) used
b)
the volume of NaOH (aq) used
c)
the number of moles of HCl (aq) used
What type of reaction is occurring in the lab? (MC 1 mark)
Based on your answers to questions 2c and 3, what much the number of moles of NaOH (aq)
required for titration be for Trial #1? (MC 3 marks)
What is the concentration of the unknown NaOH (aq) solution based on your info from Trial #1?
(MC 2 marks)
Calculate the average concentration of NaOH (aq) from the 3 trials. Show all of your work for
calculating this average. (MC 3 marks)
List 3 acceptable sources of error for this lab. Discuss how each might affect the calculated value
of NaOH (aq). (C 3 marks)
Write a brief concluding statement.
SCH3U
K
Acid Base Titration Pre-Lab
/ 11
TI
Name: ______________________
/5
Prelab questions and calculations:
1.
After reading the entire lab, ensure that you do not have any questions about the lab or lab
procedure.
2.
Define the term hygroscopic. Indicate why this is key to changing the NaOH (aq) concentration in
the lab. ( K 2 marks)
3.
Write a balanced chemical equation for the synthesis reaction between carbon dioxide gas and
water. Name the weak acid that is produced. ( K 3 marks)
4.
Write the reaction between the product of question 3 and NaOH (aq). How will this affect the pH
of the sodium hydroxide solution - raise it or lower it- and why? ( K 4 marks)
5.
Define the term endpoint. Indicate when this occurs in a titration experiment by including the
term 'moles'. ( K 2 marks)
6.
For the following information provided in a mock trial, perform the following calculations:
(TI 5 marks)
Initial volume of HCl (aq)
13.45 mL
Initial volume of NaOH (aq)
0.00 mL
Final volume of HCl (aq)
28.60 mL
Final volume of NaOH (aq)
12.85 mL
Volume of HCl (aq) used
________
Volume of NaOH (aq) used
_________
Concentration of HCl (aq)
0.100 mol /L
# moles of HCl (aq) [n = C x V]
_________ mol
thus # moles of NaOH (aq) at the end point = __________________ mol
concentration of NaOH (aq) [C = n / V ] = _________________ M
7.
If you have any questions about the lab, please write them in the space below. Remember to ask
them in the lab.