Introduction: Simulated Lab 3a: Standardizing an Unknown Acid
Titrations provide a method of quantifying the concentration of an unknown solution. In an acid-base
titration, this is done by delivering a titrant of known concentration into an analyte of known volume
(whose concentration is unknown). Titration curves (graphs of volume vs. pH) have characteristic
sigmoidal shapes. The graph can be used to determine the strength or weakness of an acid or base.
The equivalence point of a titration is the point where the analyte has been completely consumed by
the titrant. On a Titration curve, the equivalence point is where the pH changes rapidly over a small
volume of titrant. Its exact position occurs at the inflexion point of the sigmoidal curve.
In an actual lab you rarely create titration curves. Doing so would require a pH meter (expensive) and
would also require you to record data and plot it (tedious & time consuming). It is more common to
use an indicator that changes color at or near the equivalence point.
In this lab simulation you will standardize a sample containing hydrochloric acid of unknown
concentration. You will do this by titrating the sample with a NaOH standard (1.0M NaOH). It’s
called a standard because we will be using it to measure the concentration of the unknown. In other
words, we are using the NaOH of known concentration as our “measuring stick” to measure the
concentration of acid. It’s our standard of measurement.
HCl
+
NaOH
H2O
+
NaCl
The titration curve above is for the titration of 20ml of an unknown HCl solution. The y-axis is pH
and the x-axis is the volume of 1.0M NaOH added. Notice that the pH is low at the beginning of the
titration and rises slowly as more NaOH is added. This is because there is excess HCl. The pH rises
slowly as NaOH is added and HCl is consumed. At 17-22 ml of NaOH the pH rises sharply. This
occurs because all of the HCl has been consumed and there is now an excess of NaOH. The midpoint
of the “s shaped” curve is the equivalence point. That is where concentration of acid and base are
equal. In this case, the equivalence point is at 20ml of 1.0M NaOH.
With this information we can calculate the concentration of HCl of the unknown.
At equivalence point:
Mol NaOH = mol HCl
Mol NaOH:
Mol NaOH = VNaOHMNaOH
= (0.02L) x (1.0M NaOH)
= 0.02 mol NaOH
Concentration of UNKNOWN HCl Solution:
0.02 mol NaOH = 0.02 mol HCl
notice that the volume is in Liters
Concentration(M) HCl = (mol HCl)/VHCl init
= 0.02 mol HCl/0.02L
= 1.0 mol HCl/L = 1.0M HCl
notice that volume is in Liters
We will also standardize a solution containing an unknown concentration of NaOH. For this we will
use a HCl standard of known concentration (1.0M HCl).
NaOH
+
HCl
H2O
+
NaCl
The titration curve above is for the standardization of 20ml of a NaOH solution of unknown
concentration. The x-axis plots the volume of 1.0M HCl standard used in the titration. Notice how the
pH starts out high and the pH goes down as HCl is added. It starts out high since NaOH and decreases
as -OH is consumed. In this example, the endpoint occurs at 10.0ml of 1.0 HCl. Calculation of the
NaOH solutions molarity is exactly the same as before.
At equivalence point:
Mol HCl = mol NaOH
Mol NaOH:
Mol NaOH = VHClMHCl
= (0.01L) x (1.0M HCl)
= 0.01 mol HCl
Concentration of UNKNOWN HCl Solution:
0.01 mol HCl = 0.01 mol NaOH
notice that the volume is in Liters
Concentration(M) HCl = (mol NaOH)/VNaOH init
= 0.01 mol HCl/0.02L
= 0.5 mol HCl/L = 0.5M NaOH
notice that volume is in Liters
Procedure
Standardization of an Unknown HCl Solution:
1. Select a 100ml beaker. Right click on the beaker and choose “chemicals”. From the chemical
menu, select and “unknown-HCl” and add 20ml to your beaker.
2. Record your “unknown-HCl” number in the Observations section.
a. Right click on the beaker and select:
b. Indictator-phenolphthalien-3 drops
c. pH meter
d. record titration data
e. view titration data
3. From the Equipment menu, select a 50 ml burette.
4. Right click on the burette and select “chemicals”. Add 50 ml of the 1.0M NaOH standard.
5. Titrate your unknown acid.
a. Place the burette over the beaker.
b. Set the rate of addition to ~0.1ml/ sec.
c. Watch it go.
d. At the end point, the indicator should turn red.
6. Calculate the Molarity of your HCl solution.
Standardization of an Unknown NaOH Solution:
1. Select a 100ml beaker. Right click on the beaker and choose “chemicals”. From the chemical
menu, select and “unknown-NaOH” and add 20ml to your beaker.
2. Record your “unknown-NaOH” number in the Observations section.
a. Right click on the beaker and select:
b. Indictator-methyl Blue-3 drops
c. pH meter
d. record titration data
e. view titration data
3. From the Equipment menu, select a 50 ml burette.
4. Right click on the burette and select “chemicals”. Add 50 ml of the 1.0M HCl standard.
5. Titrate your unknown Base.
a. Place the burette over the beaker.
b. Set the rate of addition to ~0.1ml/ sec.
c. Watch it go.
d. At the end point, the indicator should turn blue.
6. Calculate the Molarity of your NaOH solution.