Need help finding the molarity concentration of CoCl 2 •6H 2 O using the Beer-La

ID: 525300 • Letter: N

Question

Need help finding the molarity concentration of CoCl2•6H2O using the Beer-Lambert equation. So far we have tested 3 of samples of the cobalt solution with a spectrophotometer. Here is a table explaining the results.

Then after that we did a titration experiment with our unknown. We used 3 different volumes of a 0.0204 M EDTA solution to determine what volume of the unkown solution needed for titration. The amounts are shown below:

1. Used 2 mL of unknown solution, color change took place when 14.9 mL of EDTA solution mixed with unknown

2. Used 2.33 mL of unknown solution, color change took place when 17.34 mL of EDTA solution mixed with unknown

3. Used 2.67 mL of unknown solution, color change took place when 20.05 mL of EDTA solution mixed with unknown

My professor says we are supposed to get the concentration from the volume of EDTA but i cannot figure it out

Solution # mass (g) absorbance wavelength(m) 1 0.503 0.514 509.9 uknown solution unknown mass 0.543 509.9 2 0.665 0.679 509.9

Explanation / Answer

First determine the molar mass of CoCl2.6H2O.

Molar mass of CoCl2.6H2O = (1*58.933 + 2*35.453 + 6*2*1.008 + 6*15.9994) g/mol = 237.9314 g/mol.

For the first part of the problem, where you use Beer’s law, we need the volume of the solution to find out the concentrations. You haven’t provided the volume, hence we cannot proceed further.

For the second part, we will use the titrimetric method. The equation that we must work with is

CoCl2.6H2O (aq) + EDTA (aq) -------> CoEDTA (aq) + 6 H2O (l) + 2 Cl- (aq)

As per the balanced stoichiometric equation,

1 mole EDTA = 1 mole CoCl2.6H2O

Let us work out the first trial in details.

Moles of EDTA used = (volume of EDTA added)*(concentration of EDTA) = (14.9 mL)*(0.0204 M) = 0.30396 mmole.

As per the balanced equation, moles of CoCl2.6H2O = 0.30396 mmole.

Concentration of CoCl2.6H2O = (moles of CoCl2.6H2O)/(volume of solution) = (0.30396 mmole)/(2 mL) = 0.15198 mol/L 0.152 M (ans).

Complete the table as below:

Trial

Mmoles of EDTA = (volume of EDTA)*(concentration of EDTA)

Concentration of CoCl2.6H2O = (mmoles of EDTA)/(volume of unknown)

0.30396 mmole

0.15198 M

(17.34 mL)*(0.0204 M) = 0.353736 mmole

(0.353736 mmole)/(2.33 mL) = 0.15182 M

(20.05 mL)*(0.0204 M) = 0.40902

(0.40902 mmole)/(2.67 mL) = 0.15319 M

Average concentration of CoCl2.6H2O = 1/3*(0.15198 + 015182 + 0.15319) M = 0.15233 M 0.152 M (ans).