I tried these four problems. I am giving my answers. What did I do wrong? Could
ID: 826856 • Letter: I
Question
I tried these four problems. I am giving my answers. What did I do wrong? Could someone help.
I will award points quickly. I got behind in class due to hospital stay from accident
For number 2 and 3 I used pka + log (base/acid) to get answer
If the pKa is 1.689 what is the value of Ka? (Note: do not use scientific notation)
Answer Given: 0.020
I used Ka= 10^-pKa
Find the pH of a solution that has a weak acid concentration of 0.74 M and a weak base concentration of 0.85 M, and a pKa of 4.63 .
Answer Given: 4.77
If 10.90 mL of an Ammonia solution required 10.48 mL of 0.9723 M HCl to reach the endpoint, what's the concentration of the Ammonia solution?
Answer Given: 5.0
Find the pH of a solution that has a weak acid concentration of 1.20 M and a weak base concentration of 1.12 M, and a pKa of 6.89 .
Answer Given: 6.82
Explanation / Answer
1). If the pKa is 1.689 what is the value of Ka?
We know pKa is simply the -log(Ka), so the easiest way is to solve as shown:
pKa = -log(Ka)
1.689 = -log(Ka)
-1.689 = log(Ka)
10^-1.689 = Ka
Ka = 0.0204
2).Find the pH of a solution that has a weak acid concentration of 0.74 M and a weak base concentration of 0.85 M, and a pKa of 4.63.
Use the henderson-hasselbach equation:
pH = pKa + log(base/acid)
pH = 4.63 + log(0.85/0.74)
pH = 4.69
3). If 10.90 mL of an Ammonia solution required 10.48 mL of 0.9723 M HCl to reach the endpoint, what's the concentration of the Ammonia solution?
Use the equation M1*V1 = M2*V2
Let 1 be the ammonia and 2 be the HCl
M1*V1 = M2*V2
M1*10.9 = 0.9723*10.48
M1 = (0.9723*10.48)/10.9
M1 = 0.9348
This works because we are titrating a strong base with a strong acid, so they fully dissociate
4). Find the pH of a solution that has a weak acid concentration of 1.20 M and a weak base concentration of 1.12 M, and a pKa of 6.89 .
Here we use the henderson-hasselbach equation again.
pH = pKa + log(base/acid)
pH = 6.89 + log(1.12/1.2)
pH = 6.86
Most of your answers were close, but there may have been a calculation error