A stock solution of 0.100 \(\mathrm{M}\) cobalt (II) chloride is used to create several solutions, indicated in the data table below: \(\begin{array}{|c|c|c|}\hline \text { Sample } & {\text { Volume } \mathrm{CoCl}_{2}} & {\text { Volume }} \\ \hline & {(\mathrm{mL})} & {\mathrm{H}_{2} \mathrm{O}(\mathrm{mL})} \\ \hline 1 & {20.00} & {0} \\\ \hline 2 & {15.00} & {5.00} \\ \hline 3 & {10.00} & {10.00} \\ \hline 4 & {5.00} & {15.00} \\ \hline\end{array}\) (a) In order to achieve the degree of accuracy shown in the table above, select which of the following pieces of laboratory equipment could be used when measuring out the CoCl_{2} : \(150-\mathrm{mL}\) beaker \(\quad 400-\mathrm{mL}\) beaker \(\quad 250-\mathrm{mL}\) Erlenmeyer flask \(\begin{array}{ll}{\text { 50-mL buret }} & {\text { 50-mL graduated }} \\ {} & {\text { cylinder }}\end{array} \quad 100\) -mL graduated cylinder (b) Calculate the concentration of the CoCl, in each sample. The solutions are then placed in cuvettes before being inserted into a spectrophotometer calibrated to 560 \(\mathrm{nm}\) and their values are measured, yielding the data below: \(\begin{array}{|c|c|}\hline \text { Sample } & {\text { Absorbance }} \\\ \hline 1 & {0.485} \\ \hline 2 & {0.364} \\ \hline 3 & {0.243} \\ \hline 4 & {0.121} \\ \hline\end{array}\) (c) If gloves are not worn when handling the cuvettes, how might this affect the absorbance values gathered? (d) If the path length of the cuvette is \(1.00 \mathrm{cm},\) what is the molar absorptivity value for \(\mathrm{CoCl}_{2}\) at 560 \(\mathrm{nm}\) ? (e) On the axes on the next page, plot a graph of absorbance vs. concentrrion. The \(y\) -axes scale is set, and be sure to scale the \(x\) -axes appropriately (f) What would the absorbance values be for \(\mathrm{CoCl}_{2}\) , solutions at the following concentrations? (i) 0.067 (ii) 0.180 \(\mathrm{M}\)

Step by step solution

01

Identifying suitable laboratory equipment

As for the first part of the problem, only piece of equipment which has a capability of accurate measurement below 20.00 mL is the 50-mL buret and the 50-mL graduated cylinder. The others can't give accurate measurement and hence won't serve for this purpose. Therefore, 50-mL buret or 50-mL graduated cylinder could be used to measure out the CoCl2.

02

Calculation of concentration for each sample

For the second part, we determine the concentration of each solution using the formula \( M1 * V1 = M2 * V2 \), where \( M1 = 0.1 \) (Molarity of parent solution), \( V1 = Volume CoCl_{2} \) (Volume of parent solution in mL), and \( V2 = Volume CoCl_{2} + Volume H_{2}O \) (Total volume in mL). This will give \(M2\), the molarity of the resulting solution.

03

Impact of handling cuvettes without gloves

In the third part, we note that not wearing gloves while handling the cuvettes could affect the absorbance values as fingerprints or other residues on the cuvettes could interfere with the spectrophotometer's ability to accurately measure the absorbance.

04

Calculate molar absorptivity value

In the fourth part, we calculate the molar absorptivity value using the Beer-Lambert Law, which is \( A = εcl \), where \( A \) is absorbance, \( ε \) is the molar absorptivity, \( c \) is concentration, and \( l \) is the path length. The molar absorptivity can then be calculated by \( ε = A / (c \) * \( l) \).

05

Plot a graph of absorbance vs. concentration

For the fifth part of the problem, graph the points from the given data (concentration and absorbance). This should be a linear graph as per Beer's law, which states that absorbance is directly proportional to concentration.

06

Predict the absorbance values for given concentrations

Finally, use the graph plot to predict the absorbance values for the given concentrations.

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