In: Chemistry
A.) What is the pH of a buffer prepared by adding 0.607 mol of the weak acid HA to 0.406 mol of NaA in 2.00 L of solution? The dissociation constant Ka of HA is 5.66×10−7. ANSWER IS: 6.072
B.) What is the pH after 0.150 mol of HCl is added to the buffer from Part A? Assume no volume change on the addition of the acid. ANSWER IS: 5.776
C.) What is the pH after 0.195 mol of NaOH is added to the buffer from Part A? Assume no volume change on the addition of the base.
I had to request the answer for part B but I don't know how to work the problem out. Please help me!
A.) What is the pH of a buffer prepared by adding 0.607 mol of the weak acid HA to 0.406 mol of NaA in 2.00 L of solution? The dissociation constant Ka of HA is 5.66×10−7. ANSWER IS: 6.072
Solution :- Weak acid and its conjugate base moles are given and asked to calculate the pH
Lets first calculate the pH of the solution using the Henderson equation
pH= pka + log ([base]/[acid])
pka = -log ka
= -log 5.66*10^-7
= 6.247
Now lets put the values in the formula
pH= 6.247 + log [0.406/0.607]
pH= 6.247+(-0.175)
pH= 6.072
B.) What is the pH after 0.150 mol of HCl is added to the buffer from Part A? Assume no volume change on the addition of the acid. ANSWER IS: 5.776
Solution :-
After adding the 0.150 mol of HCl the acid moles will increase and base moles will decrease by the same amount
Therefore new moles of acid = 0.607 +0.150 = 0.757 mol
New moles of base = 0.406-0.150 = 0.256 mol
Now lets calculate the pH using the same formula
pH= 6.247 + log [0.256/0.757]
pH= 6.247+(-0.471)
pH= 5.78
C.) What is the pH after 0.195 mol of NaOH is added to the buffer from Part A? Assume no volume change on the addition of the base.
Solution :-
After adding the NaOH the moles of acid will decrease and moles of base will increase
So new moles of acid = 0.607 – 0.195 = 0.412 mol
Moles of base = 0.406 +0.195 = 0.601 mol
Now lets calculate the pH
pH= 6.247 + log [0.601/0.412]
pH= 6.247+ 0.164
pH= 6.41