PRE-LAB EXERCISE FOR LAB 7: Simulated Acid Base Titration Pre Lab questions: For
ID: 1017324 • Letter: P
Question
PRE-LAB EXERCISE FOR LAB 7: Simulated Acid Base Titration
Pre Lab questions:
For this pre lab assignment you will be playing the role of the instructor. On some of the questions below you will find some previous answers that have been submitted for this exercise. Below the given answer, clearly state what is wrong with the answer(s) given and explain what the correct answer should be. For those questions with no answer submitted, clearly state the correct answer and give your reasoning
1. Refer back to the definitions of acids and bases and indicate the role of water in the example below? What is the role of H3O+? (Your correction should address all three answers below.)
HA+H2O A- +H3O+
Answer #1: In this example water is accepting an H+ making it the base is this equation. In this example H3O+ is a conjugate acid of water.
Answer #2: The role of water is to help ionize the acid and the role of the H30+ is to determine the strength of the weak acid by the ratio of hydronium ion to the conjugate base.
Answer #3: The H3O+ is acting as the reactant.
What is the difference between a strong base and a weak base? Give an example of a strong base.
Answer: A strong base will ionize 100% in water, where as a weak base is will partially ionize in water. An example of a strong base is HCl.
What do you think the pH will be at the equivalence point with the titration of a weak acid and a strong base? What about a strong acid and a weak base? Explain your reasoning.
Answer: In a weak acid and strong base, the equivalence point will be at the neutral pH of 7.0. The same would go for a strong acid and weak base. The equivalence point is supposed to be when the acid and base neutralize each other evenly, and this middle area is 7.0.
Do you expect the actual concentration of the sodium hydroxide in solution to be higher or lower than what you would calculate from the mass of dissolved sodium hydroxide in solution? Explain your reasoning.
No answer given. Give your answer with a full explanation.
Which is the stronger acid, one with Ka equal to 1 x 10-5 or one with Ka equal to 1 x 10-9?
Answer: The stronger acid is the Ka equal to 1 x 10-5 because it has a pH value of 5.
6. Why is the volume of the strong base added on the x-axis in the titration plot?
Answer: The volume is added on the x-axis because then you can determine how much base was needed to neutralize the acid
7. In a standardization of the NaOH solution, you found that a beaker with 25.0 mL of water with 5.55 g of KHP dissolved required 17.25 mL of NaOH to reach the equivalence point. How many moles of NaOH are in the solution? What is the actual concentration of the original NaOH solution? If an extra 25.0 mL of water were added to the NaOH solution before the titration with KHP, would more, less or the same amount of KHP have to be added to reach the equivalence point? (The molar mass of KHP is 204.22 g/mol)
No answer given. Give your answer and show all the steps you took.
Based on what you know predict which of the three amino acides (L-Alanine, Glycine, L- Aspertic Acid) will have the lowest pH at the equivalence point? Which will have the highest?
No answer given. Give your answer and provide reasoning.
PLEASE ANSWER THESE QUESTIONS!
Explanation / Answer
1) In the reaction HA + H2O -------> A- + H3O+, water molecule accepts a proton to form hydronium ion, H3O+. Thus, water can be viewed as a Bronsted base since it accepts a proton from HA to form the conjugate acid, H3O+. The pH of the solution is given by the concentration of H3O+ in the solution; H3O+ carries the same charge as a proton, but a bare proton, i.e, H+ cannot exist in solution and is always picked up by a water molecule to form H3O+. We can also say that H3O+ is one of the products of the acid base reaction between the HA (acid or proton donor as per Bronsted-Lowry theory) and water (base or proton acceptor as per the same theory).
2) A strong base is usually an ionic base that splits upto into the metal ion and hydroxide ion in an aqueous solution. As has been mentioned in the answer already, there is 100% dissociation for a strong base. Usually bases of alkali and alkaline earth metals are strong bases, eg., sodium hydroxide (NaOH), magnesium hydroxide [Mg(OH)2], etc. The dissociation of an aqueous solution of a strong base like NaOH can be shown as
NaOH (s) ---------> NaOH (aq) ---------> Na+ (aq) + OH- (aq)
The pH of the solution is given as pH = -log10[H+]; however, the above reaction produces no hydrogen ion, but hydroxide ion (basic). So, we need to calculate the pOH of the solution instead as pOH = -log10[OH-] (note that sodium ion plays no role here).
We can then calculate the pH by employing the relation pH + pOH = 14.
A weak base, on the contrary is one that doesn’t undergo complete dissociation in aqueous solution. A good example is ammonia, NH3.
NH3 + H2O <=====> NH4+ + OH-
The dissociation reaction is an equilibrium reaction, since complete ionization of the base doesn’t occur. Th equilibrium constant for the dissociation reaction is
Kb = [NH4+][OH-]/[NH3][H2O]
Kb is called the base dissociation constant. [H2O] is usually quite large and constant in an aqueous solution, so we ignore that and write a simplified expression for Kb as
Kb = [NH4+][OH-]/[NH3] (we do not ignore the [NH4+] here as it is a protonated species).
Calculation of pH of the solution isn’t very straightforward here and we must first find out Ka (dissociation constant of the conjugate acid by using the auto-protolysis of water reaction) and then compute pH.
3) The pH at the equivalence point of a weak acid/strong base titration is not 7 unlike the case of a strong acid/strong base titration. The pH is usually higher than 7. Consider a strong acid/strong base titration like
HCl + NaOH ------> NaCl + H2O
At the equivalence point, the moles of strong acid = moles of strong base. NaCl is a salt and plays no parts in titration. The pH at the end point is thus governed by the pH of water, which is 7.
Now consider a weak acid like acetic acid, CH3COOH and a strong base like NaOH.
CH3COOH + NaOH ------> CH3COO-Na+ + H2O
Offcourse, water is still there, but we also have sodium acetate, CH3COO-Na+. Acetate ion, CH3COO- is the conjugate base of acetic acid (Bronsted-Lowry theory) and will react with water (present in large quantities in the aqueous solution) as
CH3COO- + H2O <====> CH3COOH + OH-
The acetate ion produces hydroxide ions at the end point on reaction with water. It is because of the formation of hydroxide ions that the pH is usually higher than 7. The exact pH of the solution will be governed by the concentrations of acetate ion and hydroxide ion in the solution at the end point.
Similarly, the pH at the equivalence point of a strong acid/weak base titration will not be 7. Consider the titration of the weak base NH4OH with the strong acid, HCl.
NH4OH + HCl ------> NH4Cl + H2O
NH4Cl contains NH4+, the conjugate acid of NH3. NH4+ establishes equilibrium with water as
NH4+ + H2O <======> NH4OH + H+
Note that here, H+ is produced; the pH of the solution at the equivalence point is usually lower than 7. The actual pH will be governed by the concentrations of NH4+ and H+.
4) The actual concentration of sodium hydroxide in solution will be lower than that calculated from the mass of sodium hydroxide. The reason is that sodium hydroxide is highly ionic and some of it is dissociated into ions even in the solid state. Thus, what we are considering as solid sodium hydroxide is actually a combination of some sodium ions, some hydroxide ions and the remaining undissociated sodium hydroxide. It is due to the tendency of such electrolytes to ionize in the solid state that a more correct way of expressing concentration of sodium hydroxide is by the term “formula weight” which is defined as the formula mass of sodium hydroxide that undergoes dissolution. Formula mass is the combination of the dissociated and undissociated forms.
5) Two acids have Ka 1.0*10-5 and 1.0*10-9. Usually a good rule of thumb is that lower the Ka value, the weaker the acid. Thus, the acid with Ka value of 1.0*10-5 is the stronger acid. The reason can be easily understood if we look at how Ka is defined. For a weak acid HA that dissociates as HA -----> H+ + A-,
Ka = [H+][A-]/[HA]
A higher Ka value means higher [H+], i.e, the acid has a greater tendency to donate protons. Since the proton donation capability of a strong acid is taken as a measure of its acid strength, the acid with the higher Ka value will be a stronger acid.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.