Experiment 5: Acid-Base Equilibrium: Capacity of Buffers
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32 Terms
What is the purpose of the experiment discussed in the notes?
To make buffer solutions at a desired pH, test their reactions to strong acid and strong base, and analyze how buffer capacity is affected by pH and concentrations.
What is a buffer solution?
A solution containing significant amounts of both a weak acid and its conjugate base OR a weak base and its conjugate acid.
What is the Henderson-Hasselbalch Equation?
pH = pKa + log([A-]/[HA]) used to estimate the pH of buffer solutions.
What defines a strong acid?
An acid that dissociates completely in water.
Give examples of strong acids.
HCl, HBr, HI, HNO3, HClO4, H2SO4.
What is the relationship between acid dissociation constant (Ka) and acid strength?
The greater the value of Ka, the more an acid dissociates in water.
How do weak acids behave in water?
They donate a proton to water (Bronsted-Lowry definition) and do not dissociate completely.
What is buffer capacity?
The ability of a buffer solution to resist changes in pH upon the addition of strong acid or base.
What formula represents buffer capacity?
Buffer Capacity = ΔVadded / ΔpHsolution.
What two methods are outlined for making a weak acid buffer solution?
Using a weak acid and adding conjugate base.
Using a weak acid and adding strong base.
What pH value should the acetic acid/acetate buffers be made at?
Buffer solutions should be prepared at the pH assigned by the TA.
What are the components of Buffer 1 in the experiment?
0.50 M acetic acid and 0.50 M sodium acetate.
What are the components of Buffer 2 in the experiment?
0.50 M acetic acid and 0.50 M sodium hydroxide.
What should be done with the pH electrode during the experiment?
Lightly blot with tissue instead of wiping and immerse in at least an inch of solution.
What is the initial step to titrate the buffers?
Record the initial pH of the buffer solutions.
How do you determine buffer capacity through titration?
By titrating with 0.50 M NaOH or HCl until the pH of the solution changes by 2 pH units.
What is the change in pH that is measured during titration?
All titrations should be measured in increments of 0.2 pH units.
What happens to buffer capacity at the pH equal to pKa?
The buffer has maximum capacity when the concentrations of the weak acid and its conjugate base are equal.
How does buffer capacity compare when titrated with NaOH versus HCl?
An acetic acid/acetate buffer has a higher capacity when titrated with NaOH than with HCl.
What is a common source of error in the experiment?
Rounding during the calculation of the ratio of component volumes needed to make the buffer solutions.
What must be done to the pH meter between measuring different solutions?
Rinse and blot the electrode to avoid contamination.
What is the significance of pH and concentrations when making a buffer?
Both must be taken into account to ensure proper buffer capacity.
Describe how strong bases dissociate in water.
Strong bases dissociate completely in water, particularly those containing alkali metals.
What should be done to ensure accurate pH measurements with a pH meter?
Know how to calibrate the meter, make accurate measurements, and store it properly.
Why is it important to understand buffer solutions in chemistry?
They help resist changes in pH in various biological and chemical processes.
What type of measurement is crucial prior to titration?
Initial pH measurement of the buffer solutions.
What is the impact of adding excess strong acid/base to a buffer system?
The buffer may become overwhelmed and unable to maintain stable pH.
What does the term 'titrate' mean in the context of buffer solutions?
To incrementally add a titrant to a solution until a desired pH change is achieved.
What should the total volume of each buffer solution be when prepared?
40 mL total for each buffer.
How does the introduction of a strong acid affect a buffer solution?
It challenges the buffer by increasing H+ concentration, potentially altering the pH.
How does the introduction of a strong base affect a buffer solution?
It challenges the buffer by decreasing H+ concentration, potentially altering the pH.
Why is it important to understand the pKa of an acid when working with buffer solutions?
The buffer is most effective within ±1 pH unit of the pKa of the acid.