Chapter 11: Q75P (page 263)

A certain acid-base indicator exists in three colored forms:
The units of molar absorptivity, ε, are M^-1 cm^-1. A solution containing 10.0 mL of 5.00 × 10^-4 M indicator was mixed with 90.0 mL of 0.1 M phosphate buffer (pH 7.50). Calculate the absorbance of this solution at 435 nm in a 1.00-cm cell.

Short Answer

Expert verified

The absorbance of the solution will be 0.815

Step by step solution

Information given

Volume of phosphate buffer=90mL

Volume of the solution containing indicator= 10mL

Concentration of the indicator=5.00 ×10^-4 M

Concentration of phosphate buffer= 0.1 M

pH=7.5

Wavelength =435 nm

Determine the concentration of In2-

At pH 7.5 , presence of H₂In in solution is negligible. As it can be seen from the given reaction that pK₁=1.00, therefore we can write

$\begin{matrix} [H I n^{-}] + [I n^{2 -}] = 5.00 \times 10^{- 5} \\ p H = p K_{2} + \log \frac{[I n^{2 -}]}{[H I n^{-}]} \\ 7.5 = 7.95 + \log \frac{[I n^{2 -}]}{5.00 \times 10^{- 5} - [I n^{2 -}]} \\ [I n^{2 -}] = 1.31 \times 10^{- 5} M \end{matrix}$

$\begin{matrix} [H I n^{-}] = 5.00 \times 10^{- 5} - [I n^{2 -}] \\ = 3.69 \times 10^{- 5} M \end{matrix}$

Determine the absorbance

Let the absorbance can be denoted by the symbol A

$\begin{matrix} A_{λ} = ε C L \\ C_{I} = c o n c e n t r a t i o n o f t h e s u b s \tan c e \\ L = L e n g t h o f t h e c e l l \end{matrix}$

Therefore, we can write

$\begin{matrix} A_{435} = ε_{435} [C_{H I n^{-}}] (1.00) + ε_{435} [C_{I n^{2 -}}] (1.00) \\ = ε_{435} [H I n^{-}] (1.00) + ε_{435} [I n^{2 -}] (1.00) \\ = [(1.80 \times 10^{4}) \times (3.69 \times 10^{- 5}) \times (1.00)] + [(1.15 \times 10^{4}) \times (1.31 \times 10^{- 5}) \times (1.00)] \\ = 0.815 \end{matrix}$

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Short Answer

Step by step solution

Information given

Determine the concentration of In2-

Determine the absorbance

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A certain acid-base indicator exists in three colored forms:The units of molar absorptivity, ε, are M-1 cm-1. A solution containing 10.0 mL of 5.00 × 10-4 M indicator was mixed with 90.0 mL of 0.1 M phosphate buffer (pH 7.50). Calculate the absorbance of this solution at 435 nm in a 1.00-cm cell.

Short Answer

Step by step solution

Information given

Determine the concentration of In2-

Determine the absorbance

One App. One Place for Learning.

Most popular questions from this chapter

Recommended explanations on Chemistry Textbooks

Nuclear Chemistry

Chemical Reactions

Ionic and Molecular Compounds

Inorganic Chemistry

The Earths Atmosphere

Chemical Analysis

Study anywhere. Anytime. Across all devices.