What concentration of HCl will have a pH of 5.0?
b) What concentration of acetic acid will have a pH of 5.0? (For acetic acid, pKa=4.76)
c) What concentration of phosphoric acid will have a pH of 5.0? (For H3PO4, pKa1 = 2.12, pKa2 = 7.21, pKa3 =12.32)
d) What is the pH of a 10 mM solution of phosphoric acid?
e) How much 1.0 M NaOH must be added to 100 ml of 10 mM H3PO4 to raise the pH of the solution to 7.0?
f) If 2 volumes of 0.2 M Na2HPO4 are added to 1 volume of 0.1 M H3PO4, what is the pH of the resulting phosphate buffer?
(a) HCl is strong acid . [H+] = [HCl]
pH = 5.0 = -log[H+]
[HCl] = 10-5.0 = 1.0×10-5M (answer)
(b) acetic acid is a weak acid, for weak acid .
Ka = 10-4.76 = 1.74×10-5 = [H+] × /(1-) = 1.0×10-5× /(1-)
= 1.74/2.74 = 0.63474
[H+] = c ×
c = 1.0×10-5 /0.63474 = 1.575×10-5 M
Concentration of acetic acid = 1.58×10-5 M. (Answer)
(c) phosphoric acid is weak acid and 2nd dissociation constant is very small. We can consider concentration of H+ from 1st dissociation of acid only.
Ka1= 10-2.12 = 1.0×10-5( /1-)
= 0.9987
[H+] = c
Concentration of phosphoric acid = 1.0×10-5/0.9987
= 1.0×10-5 M. (Answer)
(d)
c = 10mM = 1×10-2M
Ka1 = c2/(1-)
2/(1-) = 0.7586
= 0.57068
[H+] = c = 1×10-2 × 0.57068 M
pH = -log(5.7068×10-3) = 2.244
pH = 2.24. (Answer)
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