An acid HTS is added to 1M acetic acidacetate buffer pH 4 to

An acid, HTS, is added to 1M acetic acid/acetate buffer (pH 4) to yield a 0.001M solution. HTS does not absorb whereas TS. does absorb in the visible region (A-540c for l = 650nm with c is in molarity.) If the absorbance signal from the 10-M solution is 0.27, what is Ka of HTS? For an 8x10 M methyl red solution at pH 5, As2sam is 0.92. (1em cells were used to make this measurement.) The experimenters ascertained the following two facts about 7.

Solution

Question 1)

For HTS/TS- system, at pH=4 ,[HTS]=0.001M

Also,

Using Beer\'s law ,

Absorbance=e*l*C

where e=absorptivity of species(TS-)

l=path length of light=650nm=650*10^-7 cm

Given A(TS-)=540C

C=A(TS-)/540=0.27/540=0.0005M

[TS-]=0.0005M

HTS+H2O <--->TS- +H3O+

ka=[TS-] [H3O+]/[HTS]

pH=4=-log [H3O+]

[H3O+]=10^-4M

ka=(0.0005M)(10^-4M)/(0.001M)=0.00005=5.0*10^-5M

ka=5.0*10^-5M