Could someone please show me how to get the answers below? Use the information g

Question

Could someone please show me how to get the answers below?

Use the information given below to answer the following questions about a 500.0 mL solution of methyl orange, an acid base indicator with a pKa = 3.40. The absorbance measurements were performed in a glass cuvette with a path length of 1.00 cm. a. What is the concentration of the protonated and the deprotonated methyl orange species using the absorption data from a single solution containing a mixture of both species? [HA] = 0.0936 M [A] = 0.184 M b. What is the pH of this solution? pH = 3.40 + log 0.184/0.096 = 3.68

Explanation / Answer

Ans. Part 1: Beer-Lambert’s Law, A = e C L             - equation 1,              

where,

                       A = Absorbance

                       e = molar absorptivity at specified wavelength (M-1cm-1)

                        L = path length (in cm)

                        C = Molar concentration of the solute

Let the [HIn] = [HA] A molar           , and [In-] = [A-] B molar

Note: M = mol L-1  

#1. At 423.8 nm,

Total absorbance of the mixture = Abs of HA + Abs of I-

Or, 0.078 = (0.160 mol-1cm-1 L) x A M x 1.0 cm + (0.341 mol-1cm-1 L) x B M x 1.0 cm

Or, 0.078 = 0.160 A + 0.341 B

Hence, 0.160 A + 0.341 B = 0.078           - equation 1

#2. At 501.3 nm,

Total absorbance of the mixture = Abs of HA + Abs of I-

Or, 0.038 = (0.351 mol-1cm-1 L) x A M x 1.0 cm + (0.023 mol-1cm-1 L) x B M x 1.0 cm

Or, 0.038 = 0.351 A + 0.023 B

Hence, 0.351 A + 0.023 B = 0.038           - equation 2

#3. Comparing (equation 1 x 0.351) – (equation 2 x 0.160)-

            0.05616 A + 0.009691 B = 0.027378

     (-) 0.05616 A + 0.003680 B = 0.006080

                                    0.11601 B = 0.0213

                                    Or, B = 0.0213 / 0.11601 = 0.18359

Therefore, [In-] = [A-] = 0.184 M

Putting the values of B in equation 1-

            0.160 A + 0.341 (0.184) = 0.078

            Or, 0.160 A = 0.078 - 0.062744 = 0.015256

            Or, A = 0.015256 / 0.160 = 0.09535

Therefore, [HIn] = [HA] = 0.0954 M

Note: Please recheck your value of [HA] = 0.0936 M

Part 2: Using Henderson- Hasselbalch equation-

            pH = pKa + log ([A-] / [HA])

            or, pH = 3.40 + log (0.184 / 0.0954)

            or, pH = 3.40 + log (1.9287) = 3.40 + 0.29 = 3.69

Therefore, pH = 3.69

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.