Question

A 0.2 m solution of a weak acid ha dissociates such that 99.4% of the weak acid remains intact (i.e., remains as ha). what is the pka of the weak acid?

Answer 1

The pKa of a weak acid can be determined using the equation: pKa = -log10(Ka). In this case, we are given that 99.4% of the weak acid remains intact, which means that only 0.6% has dissociated. Therefore, we can calculate the equilibrium concentration of HA by multiplying the initial concentration of HA by 0.006.

Step-by-step explanation:

The pKa of a weak acid can be determined using the equation: pKa = -log10(Ka), where Ka is the acid dissociation constant. In this case, we are given that 99.4% of the weak acid remains intact, which means that only 0.6% has dissociated. Therefore, we can calculate the equilibrium concentration of HA by multiplying the initial concentration of HA by 0.006.

Since the [HA] is equal to the [HA]₀ * 0.006, we can use this calculated equilibrium concentration along with the given Ka value to find the pKa.

For example, let's assume the initial concentration of HA is 0.2 M. The equilibrium concentration of HA would be 0.2 M * 0.006 = 0.0012 M. Using the equation pKa = -log10(Ka), we can find the pKa value.

Answer 2

HA -----> H(+) + A(-)
0,2-x.........x..........x

100% - 99,4% = 0,6% = 0,006

x = 0,6%×0,2 = 0,006 × 0,2 = 0,0012

HA --------------> H(+) + A(-)
0,2-x.....................x..........x
0,2-0,0012.....0,0012..0,0012
0,1988.............0,0012..0,0012

K = [H+][A-]/[HA]
K = (0,0012)^2/0,1988
K = 0,0000072434

pKa = -logK= -log0,0000072434 = 5,14

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