For dilute solutions/low concentrations of substances there is a direct relation

Question

For dilute solutions/low concentrations of substances there is a direct relationship between Absorbance and concentration. This is known as Beer's Law, A = cbc, e = molar absorptivity (a constant), b = width of cuvette which holds the solution in the spectrophotometer and c = concentration in moles/Liter. A solution of substance x has an Absorbance of 0.345 for a concentration of 1.5 times 10^-4 m/L in a cm, cuvette. Using this information and Beer's law calculate the concentration of a solution of x which has an Absorbance of 0.2 16 in a 1 cm. cuvette. A simple salt soluble in water forms a white precipitate upon addition of silver nitrate. The supernate is removed and when made basic with ammonia a blue solution results. The precipitate dissolves on making it basic with ammonia. The precipitate reappears after making this last solution acidic with nitric acid. Tell cation and anion and/or formula of salt.

Explanation / Answer

Ans. #1. Method 1: Beer-Lambert’s Law, A = e C L             - equation 1,              

where,

                       A = Absorbance

                       e = molar absorptivity at specified wavelength (M-1cm-1)

                        L = path length (in cm)

                        C = Molar concentration of the solute

Note that absorbance of a solution is directly proportion to its concentration.

So, (Abs of soln. 1 / Abs of soln. 2) = (Conc. of soln. 1 / Conc. of soln. 2)  

Or,

            0.345 / 0.216 = (1.5 x 10-4 m/L) / C2

            Or, C2 = (1.5 x 10-4 m/L) / 1.5972 = 9.391 x 10-5 m/L

Therefore, the concertation of solution 2 = 9.391 x 10-5 m/L

Method 2: Putting the values for solution 1-

            0.345 = e x (1.5 x 10-4 m/L) x 1 cm

            Or, e = 2300 L m-1 cm-1

Thus, extinction coefficient of the molecule X = 2300 L m-1 cm-1

Now, using ‘e’ for solution 2-

            0.216 = (2300 L m-1 cm-1) x C x 1.0 cm

            Or, C = 0.216 / (2300 L m-1) = 9.391 x 10-5 m/L

Therefore, the concertation of solution 2 = 9.391 x 10-5 m/L

#2. The characteristic blue color in basic/ alanine solution is due to presence of Cu2+ ions. So, cation is Cu2+.

Cl- ions react with AgNO3 to form white, insoluble precipitate of AgCl. Thus, the anion is Cl-.

            Cl-(aq) + AgNO3(aq) -----> AgCl(s, white ppt.)

Formula of salt = CuCl2

CuCl2­ is highly soluble in water (around 75.7 g/ 100 mL at room temperature).

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