In case study 3, we showed how colorimetry can be used to...

In case study $3,$ we showed how colorimetry can be used to follow the oxidation of $\left[\mathrm{C}_{2} \mathrm{O}_{4}\right]^{2-} \mathrm{by}$ $\left[\mathrm{MnO}_{4}\right]^{-}$ ions in acidic aqueous solution. The data shown in Figure 11.8 refer to an experiment in which $2.0 \mathrm{cm}^{3}$ of aqueous $\mathrm{KMnO}_{4}\left(4.0 \times 10^{-4} \mathrm{moldm}^{-3}\right)$ are mixed with $1.0 \mathrm{cm}^{3}$ aqueous sulfuric acid solution that contains $\left[\mathrm{C}_{2} \mathrm{O}_{4}\right]^{2-}$ ions $\left(x \mathrm{moldm}^{-3}\right)$ in a cuvette of path length $1.0 \mathrm{cm}$. (a) Rationalize the shape of the curve shown in Figure $11.8 .$ (b) The initial absorbance (time $=0 \mathrm{s}$ ) is 0.65 Determine the molar extinction coefficient of $\mathrm{KMnO}_{4} \cdot(\mathrm{c})$ If the final absorbance is 0.14, calculate the concentration of $\left[\mathrm{MnO}_{4}\right]^{-}$ ions remaining in solution at the end of the reaction. (d) How many moles of $\left[\mathrm{MnO}_{4}\right]^{-}$ are used during the reaction? How many moles of $\left[\mathrm{C}_{2} \mathrm{O}_{4}\right]^{2-}$ were present initially (see equation 11.6 )? Hence determine the concentration of $\left[\mathbf{C}_{2} \mathbf{O}_{4}\right]^{2-}$ present in the original $1.0 \mathrm{cm}^{3}$ sample of oxalate ions.

In case study $3,$ we showed how colorimetry can be used to follow the oxidation of $\left[\mathrm{C}_{2} \mathrm{O}_{4}\right]^{2-} \mathrm{by}$ $\left[\mathrm{MnO}_{4}\right]^{-}$ ions in acidic aqueous solution. The data shown in Figure 11.8 refer to an experiment in which $2.0 \mathrm{cm}^{3}$ of aqueous $\mathrm{KMnO}_{4}\left(4.0 \times 10^{-4} \mathrm{moldm}^{-3}\right)$
are mixed with $1.0 \mathrm{cm}^{3}$ aqueous sulfuric acid solution that contains $\left[\mathrm{C}_{2} \mathrm{O}_{4}\right]^{2-}$ ions $\left(x \mathrm{moldm}^{-3}\right)$ in a cuvette of path length $1.0 \mathrm{cm}$. (a) Rationalize the shape of the curve shown in Figure $11.8 .$
(b) The initial absorbance (time $=0 \mathrm{s}$ ) is 0.65 Determine the molar extinction coefficient of $\mathrm{KMnO}_{4} \cdot(\mathrm{c})$ If the final absorbance is 0.14, calculate the concentration of $\left[\mathrm{MnO}_{4}\right]^{-}$ ions remaining in solution at the end of the reaction. (d) How many moles of $\left[\mathrm{MnO}_{4}\right]^{-}$ are used during the reaction? How many moles of $\left[\mathrm{C}_{2} \mathrm{O}_{4}\right]^{2-}$ were present initially (see equation 11.6 )? Hence determine the concentration of $\left[\mathbf{C}_{2} \mathbf{O}_{4}\right]^{2-}$ present in the original $1.0 \mathrm{cm}^{3}$ sample of oxalate ions.

Key Concepts

Reaction Stoichiometry

Reaction stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It involves using the balanced chemical equation to calculate the moles of reactants consumed and products formed, which is essential when relating experimental measurements to theoretical predictions.

Spectrophotometric Kinetics

Spectrophotometric kinetics involves monitoring the progress of a chemical reaction by measuring the absorbance of a reactant or product over time. Changes in absorbance reflect changes in concentration, enabling the study of reaction rates and mechanisms through a non-invasive, real-time method.

Colorimetry

Colorimetry is an analytical technique used to determine the concentration of colored compounds in solution by measuring the intensity of light absorbed. It relies on the principle that the concentration of a substance is directly related to its ability to absorb light at a specific wavelength.

Beer-Lambert Law

The Beer-Lambert Law provides a linear relationship between absorbance and concentration, stating that absorbance is proportional to the path length, the molar absorptivity (extinction coefficient), and the concentration of the absorbing species. This law is fundamental in converting absorbance data into quantitative concentration information.

Transcript

00:01 Okay, so this is about reaction between the potassium manganese oxide and the c2 -04 minus 2 irons.

00:14 And basically what they do is actually the mix of the concentration of c -204 with the manganese oxide.

00:26 04 irons, right? and it states, actually the manganese oxide iron will have the absorbance.

00:42 So this is actually the figure showing the absorbance change during the reaction.

00:50 So the first question is to ask us to rationalize the shape of the curve shown.

00:57 So as we can see here, right, at the beginning, it goes down, right? which means the manganese 04 minors is consumed because absorbance goes down.

01:17 But at a certain point, it becomes flat, and this is not zero, right? it's non -zero, which means actually there are still the magnesium oxide for left.

01:34 So for the reaction, we actually know there are actually excessive manganese 04 left, and the c2 -04 irons were totally consumed.

01:49 And for the quest b is the initial absorbence is 0 .65, and ask us to determine the molar extinction coefficient of the potassium magnesium 0 .4.

02:06 So it's about beer lambda store, right? so let's first write down the beer lamber store, which is the absorbence equals to the extinction coefficient times the concentration and times the l.

02:30 So first, the initial concentration of the potassium magnesium magnesium dioxide is also given, right? but you have to be very careful about here because if you look at the question, it says there are two millimeters of the potassium oxide, right, mixed with one millimeter of the c204.

03:08 Which means, right, in the mixed solution, there are actually three millimeters of the solution.

03:18 And the initial concentration of the potassium magnesium oxide will be changed, right? so we can calculate first the initial, right, concentration of the potassium magnesium oxide in the mixed solution, which is equal to, if you remember, how to calculate the solution in the mix, c0v, the volume, equals to the original concentration where you c8 times va, right? so the volume in the mixed solution, right, is 2 milligrams plus 1, right? because it mixed with another 1 millimeters of the c204.

04:17 And for the original solution, right, the original potassium magnesium oxide solution, the concentration is 4 times 10 to minus 4.

04:33 And the volume is 2 centimeters.

04:39 And then we can get the initial concentration of the mixed solution is actually three a third times 10 to minus four more per liter and then we can actually plug in this to the blaml's law so let's just do that right so we can do an rearrangement of the blm's law which becomes the coefficient equals to the absorbence over the concentration times the pathlands.

05:34 Then the adorbents given is 0 .65 over the concentration, right? while we calculate here, it's 8 thirds times 10 to minus 4.

05:55 And then the pathlands is warm.

06:00 And you can use your calculator to do the calculation.

06:03 You'll find the answer is 2 .4 times 10 to 3 liters per more per centimeter...

Please give Ace some feedback