Consider a buffer solution consisting of CH3NH3Cl and CH3NH2. Which of the following statements are true concerning this solution?
(Ka for CH3NH3+ = 2.3 x 10 -11).
1. A solution consisting of 0.1 M CH3NH3Cl and 0.1 M CH3NH2 would be a more effective buffer than one containing 1.5 M CH3NH3Cl and 1.5 M CH3NH2
2. Adding more [CH3NH3+ to the initial buffer solution will increase the pH.
3. If [CH3NH3+] >[CH3NH2], then pH is larger than the pKa value.
4. If [CH3NH3+] < [CH3NH2], then the [H+] is larger than the Ka value.
5. If [CH3NH3+] = [CH3NH2], then pH = 7.00.
6. If NaOH were added to the initial buffer solution, then the [CH3NH3+] would decrease.
Let's go through each statement one by one:
1. A solution consisting of 0.1 M CH3NH3Cl and 0.1 M CH3NH2 would be a more effective buffer than one containing 1.5 M CH3NH3Cl and 1.5 M CH3NH2.
To determine the effectiveness of a buffer solution, we need to compare the concentration of the weak acid (CH3NH2) and its conjugate acid (CH3NH3+). In this case, the first statement is true. A buffer solution with equal concentrations (0.1 M) of CH3NH3Cl and CH3NH2 would be more effective than one with higher concentrations (1.5 M) of both.
2. Adding more [CH3NH3+] to the initial buffer solution will increase the pH.
When we add more of the conjugate acid (CH3NH3+), the solution becomes more acidic, resulting in a decrease in pH. Therefore, the second statement is false. Adding more [CH3NH3+] would decrease the pH.
3. If [CH3NH3+] > [CH3NH2], then pH is larger than the pKa value.
The pKa value is a measure of acidity. If [CH3NH3+] is greater than [CH3NH2], it means there is an excess of the conjugate acid, making the solution more acidic. Therefore, the third statement is false. If [CH3NH3+] > [CH3NH2], the pH would be less than the pKa value.
4. If [CH3NH3+] < [CH3NH2], then the [H+] is larger than the Ka value.
When [CH3NH3+] is less than [CH3NH2], it means there is an excess of the weak base (CH3NH2). In this case, the solution becomes more basic, resulting in a lower [H+] concentration. Therefore, the fourth statement is false. If [CH3NH3+] < [CH3NH2], the [H+] would be less than the Ka value.
5. If [CH3NH3+] = [CH3NH2], then pH = 7.00.
The pH of a solution depends on the ratio of [H+] and [OH-]. If [CH3NH3+] is equal to [CH3NH2], the solution is at its buffer capacity and the pH will be close to the pKa value (not necessarily 7.00). Therefore, the fifth statement is false.
6. If NaOH were added to the initial buffer solution, then the [CH3NH3+] would decrease.
Adding a strong base like NaOH to a buffer solution will cause the weak acid component of the buffer (CH3NH3+) to react with OH- ions, forming more CH3NH2. This reaction will result in a decrease in [CH3NH3+]. Therefore, the sixth statement is true.
To summarize:
- Statements 1, 6 are true.
- Statements 2, 3, 4, 5 are false.
To determine which statements are true concerning the buffer solution, let's analyze each statement:
1. A solution consisting of 0.1 M CH3NH3Cl and 0.1 M CH3NH2 would be a more effective buffer than one containing 1.5 M CH3NH3Cl and 1.5 M CH3NH2.
To evaluate the effectiveness of a buffer, we need to consider the ratio of the conjugate acid (CH3NH3Cl) to the conjugate base (CH3NH2). A buffer is most effective when the ratio of [conjugate acid]/[conjugate base] is close to 1.
In this case, the initial buffer (0.1 M CH3NH3Cl and 0.1 M CH3NH2) has a ratio of 1:1, making it an effective buffer. However, the second solution (1.5 M CH3NH3Cl and 1.5 M CH3NH2) has a higher concentration of both the acid and base, which leads to an imbalanced ratio. Therefore, statement 1 is true.
2. Adding more [CH3NH3+] to the initial buffer solution will increase the pH.
The equilibrium equation for the buffer system is:
CH3NH3+ (conjugate acid) + H2O ⇌ CH3NH2 (conjugate base) + H3O+
When we add more CH3NH3+ (conjugate acid), according to Le Chatelier's principle, the equilibrium will shift to the left to consume the added acid. This results in an increase in OH- concentration and a decrease in H3O+ concentration, leading to an increase in pH. Therefore, statement 2 is true.
3. If [CH3NH3+] > [CH3NH2], then the pH is larger than the pKa value.
The pKa value is the negative logarithm of Ka. In this case, Ka for CH3NH3+ is given as 2.3 x 10^-11. Since pKa = -log10(Ka), the pKa value can be calculated as -log10(2.3 x 10^-11).
When [CH3NH3+] is greater than [CH3NH2], it means that the solution is more acidic, and therefore, the pH will be smaller than the pKa value. Hence, statement 3 is false.
4. If [CH3NH3+] < [CH3NH2], then the [H+] is larger than the Ka value.
When [CH3NH3+] is smaller than [CH3NH2], the solution is more basic. In a basic solution, the concentration of H+ ions is lower than in an acidic solution. Therefore, the [H+] will not be larger than the Ka value. Statement 4 is false.
5. If [CH3NH3+] = [CH3NH2], then pH = 7.00.
The pH of a solution is determined by the ratio of [H+] to [OH-]. In an aqueous solution, when [H+] is equal to [OH-], the solution is neutral, and the pH is 7. In a buffer solution, the concentration of [H+] and [OH-] is not necessarily equal, even if [CH3NH3+] equals [CH3NH2]. Therefore, statement 5 is false.
6. If NaOH were added to the initial buffer solution, then the [CH3NH3+] would decrease.
When NaOH, a strong base, is added to a buffer solution, it reacts with the CH3NH3+ (conjugate acid) to form water and Na+ ions. This reaction reduces the concentration of CH3NH3+ in the solution, resulting in a decrease in [CH3NH3+]. Therefore, statement 6 is true.
In summary, statements 1, 2, and 6 are true, while statements 3, 4, and 5 are false.
I would go with 2 as true.
Use pH = pKa + log (acid)/(base) to evaluate the other statements.