Question

The absorbances of solutions containing Cr as dichromate $C{r}_2{O}_7^{-2}$in $1.0M{H}_{2}S{O}_4$were measured at $440$nm in a 1.0-cm cell.

Find the absorptivity of dichromate$\left({\mathrm{Lg}}^{-1}{\mathrm{cm}}^{-1}\right)$and the molar absorptivity $\left({\mathrm{Lmol}}^{-1}{\mathrm{cm}}^{-1}\right)$at $440$nm.

The following results were obtained:

Short answer

By the graph between absorbance and concentration, find molar absorptivity.

Step-by-step solution

Step 1. Given information

Data regarding different absorbance at different concentration is given.

step 2. Solution

First convert the concentration of chromium into dichromate ion as,

$$\begin{gathered} C_{{\mathrm{C}}_2{\mathrm{O}}_7^{2-},\mathrm{g}/\mathrm{L}}=\frac{x\mu \mathrm{gCr}}{\mathrm{mL}}\times \frac{{10}^{-6}\mathrm{gCr}}{\mu \mathrm{gCr}}\times \frac{{10}^3\mathrm{mL}}{\mathrm{L}}\times \frac{215.988{\mathrm{gCr}}_2{\mathrm{O}}_7^{2-}}{{\mathrm{molCr}}_2{\mathrm{O}}_7^{2.}}\times \frac{\mathrm{molCr}}{51.996\mathrm{lgCr}}\times \frac{1{\mathrm{molCr}}_2{\mathrm{O}}_7^{2-}}{2\mathrm{molCr}} \\ =0.002077059625x \end{gathered}$$

graph between absorbance and concentration will give the absorpivity. For conversion into molar absorptivity multiply by molar mass of dichromate ion.