Could someone please explain how they found the concentration of the basic and a

Question

Could someone please explain how they found the concentration of the basic and acidic form?

Bromocresol green is a weak acid indicator with a pKa of 4.90. When protonated the dye is yellow with a molar absorptivity of 40, 500 L mol^-1 cm^-1 at 450 nm. When deprotonated the dye is blue in color with a molar absorptivity of 75,000 L mol^-1 cm^-1 at 560 nm. a. You prepare a stock solution of 100.0 mu M bromocresol green in water at pH = 2.00. What fraction of the dye molecules is in the protonated state? 2.00 = 4.90 + log basic form/acidic form 0.13% of the molecules are in the basic form 1.3 times 10^-7 M 99.87% of the molecules are in the acidic form, 9.987 times 10^-5 M

Explanation / Answer

They made use of the Henderson Hasselbach equation:

pH = pKa + log([salt]/[acid])

Here, in this case, the salt is the basic form of the dye and acid is its acidic form.

Put the values to get:

2 = 4.9 + log([basic]/[acidic])

Thus,

[basic]/[acidic] = 0.001259 ---(I)

Also, the total conc of the solution is 100 uM

This means:

[acidic] + [basic] = 100 ---(II)

Solving (I) and (II), we get:

[acidic] = 99.87 uM and [basic] = 0.13 uM

Hope this helps !

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