Question

Given the pH and absorbance of a solution. What equation would i use to find the

1.equilibrium concentration of the base? 2. equilibrium concentration of the acid?

Answer 1

Absorbance=a* l *c

where a=molar absorptivity of the solution

          l=path length =size of cuvette

         c=concentration

So, if absorbance is known then concentration can be calculated

relative absorbance of two solutions(under similar conditions) A1/A2=c1/c2

where c1,c2 are the known concentrations (standard) and unknown concentration of two solutions

2) using pH

pH=-log[H3O+]

so [H3O+]=10^-pH, so can be found out

For any given weak acid ( say HA) that tends to hydrolyse in the solution.

the dissociation of acid or base is only partial and dependent on its Ka value,higher ka means more dissociation.The pH of the solution is the measure of the equilibrium [H3O+] ion concentration

SEE the equation,

HA +H2OA- + H3O+

ka=equilibrium const=[A-] [H3O+]/[HA]

Also ,[H3O+]=[A-]=10^-pH   (known to you as pH is known)

Equilibrium concentration of the acid=[HA] - [H3O+] ([HA is acid concentration as known to you or determined from absorbance value)

Similarly for base,

pOH=14-pH

pOH=-log[OH-]

[OH-]=10^-pOH to be calculated

BOH + H2OBH +OH-

[OH-]=[BH]=-log[OH-]

equilibrium concentration of the base=[BOH]-[OH-]