Using an extinction coefficient of 15,000 m-1cm-1, calculate the concentration (in mm) of a molecule with an absorbance value of 0.200. you can assume the standard cuvette pathlength of 1 cm.

0.013 mM

Explanation:

Our strategy to solve this question will be to utilize Beer-Lambert law:

A = ε c L where  A is the absorbance,

                           ε  is the extinction coefficient (M⁻¹cm⁻1),

                           c is the concentration (in M)

                           L  is the cuvette path length  (in cm)

so the concentration, c,  from this equation will be given by

c = A/ ( ε L)

we have all the required values, thus

c= 0.200 / ( 15000 M⁻¹ cm⁻1 x 1 cm ) = 1.3 x 10⁻⁵ M

Now we are being asked to give the concentration in mmol ( 1 mol = 1000 mmol) per liter, so we need to convert our concentration:

1.3 x 10⁻⁵ mol/L x  1000 mmol/mol =  0.013 mmol/L = 0.013 mM

Answer is: concentration of the fluoride ion is 0.0368 M.
Chemical reaction: CaF₂(s) → Ca²⁺(aq) + 2F⁻(aq).
Ksp = 2.0·10⁻⁴ M.
Ksp = [Ca²⁺] · [F⁻]².
[Ca²⁺] = x.
[F⁻] = 2[Ca²⁺] = 2x.
Ksp = x · (2x)².
2.0·10⁻⁴ = 4x³.
x = [F⁻] = ∛(2.0·10⁻⁴ ÷ 4).
[F⁻] = 0.0368 M.