Lab
pH of different concentrations of a strong acid and a strong base
Goals
 To determine the pH of different solutions using a pH probe
 To create step-wise dilutions of stock solutions and to verify the dilution factors.
 To calculate the hydrogen ion concentration, [H+], for each solution and compare those to the expected values from
the dilution factors.
Theory:
The concentration of hydrogen ions in solution, [H+], is related to its pH by the formula
[H+] = 10-pH
So a solution that has a pH of 5.6 would have a hydrogen ion concentration of 2.510-6 mol dm-3.
The uncertainty of the hydrogen ion concentration can be calculated from the uncertainty of pH. If the
uncertainty in the pH value was 0.1, use the largest and/or smallest value of pH to determine the
uncertainty for the [H+]. In the above example, pH 5.7 has a [H+] = 10 –5.7 = 2.010-6 mol dm-3, while pH
5.5 has a [H+] = 10 –5.5 = 3.210-6 mol dm-3; the uncertainty could be expressed as ± 0.710-6 mol dm-3.
The dilution factor, DF, is the ratio of the final (diluted) volume, V2, to the volume of the (concentrated)
sample, V1.
V
DF  2
V1
When 25 mL of a concentrated solution is mixed with 75 mL of water so that the final volume is 100 mL,
the ratio of the final volume to the volume of concentrated sample would be 100mL : 25 mL. The
 factor.
reduced ratio, 4 : 1, is the dilution
The concentration of a substance is the amount, n (measured in moles), of substance per unit volume, V
(in dm3), of solution, according to the equation:
n
n  [X ] V
concentration of X, [ X ]  X , therefore
V
The amount of the substance, n, remains constant for both the original and diluted solutions. So,
n1  n2

[ X ]1  V1  [ X ]2  V2

V2 [ X ]1

 DF
V1 [ X ]2
[ X ]2 
[ X ]1
DF
The concentration of the final (dilute) solution, [X]2, can be determined by dividing the original
concentration, [X]1, by the dilution factor, DF. So if the original solution had a concentration of 4.0 mol
dm-3 and the dilution factor is 4:1, then the diluted solution would have a concentration of 1.0 mol dm-3.

Materials
Chemicals
hydrochloric acid, HCl (0.010 mol dm-3)
sodium hydroxide, NaOH (0.100 mol dm-3)
tap water
Glassware
test tubes (2)
glass rod (2)
Measurement
Xplorer GLX datalogger
graduated cylinder, 10 mL  0.05 mL
graduated cylinder, 100 mL  1 mL
PASPORT chemistry sensor
dropper
pH probe
Dalzell: pH of different concentrations of a strong acid and a strong base
Procedure
Part A: Acid (Hydrochloric Acid, HCl) Dilution
Original Solution

About 2 mL of the stock 0.010 mol dm-3 HCl was transferred to a clean, dry test tube.
o The pH was determined using a pH probe and recorded Table 1.
First Dilution



Exactly 5 mL of tap water was placed in a 10-mL graduated cylinder.
Exactly 1 mL of the 0.010 mol dm-3 HCl was transferred from the test tube using a glass rod.
o Always add chemicals to water – not water to chemicals.
The 10-mL graduated cylinder containing the 1.0 mL sample of 0.010 mol dm-3 HCl was slowly filled to the 10.0 mL
mark using tap water (and a dropper if desired) to make the hydrochloric acid more dilute.
o The pH of the 10-mL solution was recorded Table 1.
Second Dilution



Exactly 50 mL of tap water was placed in a 100-mL graduated cylinder.
Exactly 1 mL of the diluted HCl (after the 1st dilution) was measured and transferred to 100-mL graduated cylinder
The 100-mL graduated cylinder containing the 1.0 mL diluted sample of HCl was slowly filled to the 100.0 mL mark
using tap water to make the hydrochloric acid even more dilute.
o The pH was determined using a pH probe while adding water.
o The pH of the 100-mL solution was recorded Table 1.
Part B: Base (Sodium Hydroxide, NaOH) Dilution

The above procedure for the hydrochloric acid was repeated using 0.100 mol dm-3 sodium hydroxide, NaOH.

The pH values were recorded in Table 2.
Data Processing



Show one sample calculation for each of the four calculated values in the Table 1.
Use the data from Part A: Acid Dilution (Table 1) for the solution After 1st Dilution
Bonus: Perform any uncertainty calculations necessary and compare the calculated uncertainties with the relative
errors
Conclusion and Evaluation





State important experimental results: values, observations, and trends.
Comment on the achievement of the goals.
Identify and evaluate sources of experimental error
Evaluate the experiment and its results
Propose practical ways to improve the investigation: accuracy, precision, efficiency, effectiveness, variations
2
May 17, 2011
Dalzell: pH of different concentrations of a strong acid and a strong base
Raw and Processed Data
Table 1. Acid (hydrochloric acid) dilution raw data (pH) and processed data. The concentration of the original solution of hydrochloric acid was labeled 0.010 mol dm-3; the uncertainty for this is
unknown and assumed to be negligible.
hydrogen ion concentration, [H+] / mol dm-3
Solution
1: Original Solution
(0.010 mol dm-3)
dilution
factor
1
pH
(raw data)
calculated from pH
expected from dilution factor
+/–
3: After 2nd Dilution
(in 100 mL cylinder)
+/–
absolute error, ∆[H+] mol dm-3
0.010
+/–
2: After 1st Dilution
(in 10 mL cylinder)
error in hydrogen ion concentration
May 17, 2011
[ H  ]
[H ]

Other observations:
3
relative error,
Dalzell: pH of different concentrations of a strong acid and a strong base
Table 2. Base (sodium hydroxide) dilution raw data (pH) and processed data. The concentration of the original solution of sodium hydroxide was labeled 0.100 mol dm -3; the uncertainty for this is
unknown and assumed to be negligible.
hydroxide ion concentration, [OH-] / mol dm-3
Solution
1: Original Solution
(0.100 mol dm-3)
dilution
factor
1
pH
(raw data)
pOH
calculated from pOH
+/–
3: After 2nd Dilution
(in 100 mL cylinder)
+/–
absolute error, ∆[OH+]
mol dm-3
0.100
+/–
2: After 1st Dilution
(in 10 mL cylinder)
expected from dilution
factor
error in hydroxide ion concentration
Other observations:
4
May 17, 2011
relative error, [OH  ]
[OH  ]