Chemistry Reference
In-Depth Information
The added strong acid reacts with the high concentration of
free base to give the weaker acid BH
; therefore little change
occurs in pH.
On addition of base (OH
),
H
2
O
The added strong base reacts with the high concentration of
salt to form the weaker base B; therefore little change occurs
in pH.
BH
OH
3
B
(a)
p
K
a
is the negative logarithm to the base 10 of
K
a
, the acid
dissociation constant for the ionisation of the molecule. p
K
a
can be used to indicate the strength of bases (or, more
exactly, the strength of the conjugate acid of the base) since
p
K
a
A1.2
p
K
b
14.
(b)
The salt formed at the end point of this titration (ephedrine
hydrochloride) will be acidic by partial hydrolysis (salt of a
weak base and a strong acid); therefore, the pH at the end
point can be given by equation (1.2):
p
K
a
log
c
where
c
is the concentration of the salt.
pH
pH
(9.6)
log 0.05
(since the volume has doubled, the concentration has
halved).
pH
5.45
(c)
A possible composition for the buffer would be a mixture of
acetic acid and a salt of acetic acid (e.g. sodium acetate).
The concentration of sodium acetate required is found by
solving the Henderson-Hasselbalch equation (equation
1.6).
[SALT]
pH
p
K
a
log —----
[ACID]
(
---
)
x
5.0
4.76
log
0.1
x
0.1738
M
sodium acetate.
