In: Chemistry
An acetic acid buffer containing 0.50 M HC2H3O2 and 0.50 M NaC2H3O2 has a pH of 4.74. What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer?
A |
4.77 |
B |
4.71 |
C |
4.68 |
D |
4.62 |
E |
none of these choices is correct |
The answer is A) 4.71
Henderson equation is
pH = pKa + log([A-]/[HA])
pKa of acetic acid is 4.75
[ A- ] = [ HC2H3O2]= 0.5M
[ HA] = [ C2H3O2-] = 0.5M
No of mole of HC2H3O2 = (0.5mol/1000ml)×100ml = 0.05
No of mole of C2H3O2- = 0.05
HCl react with conjugate base C2H3O2-
H+ + C2H3O2- -------> HC2H3O2
1mole of HCl react with 1mole of C2H3O2- to give one mole of HC2H3O2
0.0020mol of HCl react with 0.0020mole of C2H3O2- to give 0.0020 mol of HC2H3O2
No of mole of C2H3O2- = 0.05 - 0.0020 = 0.048mol
No of mole of HC2H3O2 = 0.05 + 0.0020 = 0.052mol
[ C2H3O2- ] = 0.48M
[ HC2H3O2 ] = 0.52M
Therefore,
pH = 4.75 + log( 0.48/0.50)
=4.75 - 0.034
= 4.71