Beer Lambert Law: Calculate Molar Extinction Coefficient at 405nm

[stads_29]

Homework Statement

The p-nitrophenolate ion absorbs lights at 405nm. A solution that is 74um in the p-nitrophenolate ion was diluted 1 in 2 with water.Absorbance of resulting solution at 405nm was .650. Calculate molar extinction coefficient of the p-nitrophenolate ion at 405nm. Your meant to assume water has no absorbance at 405 nm.

How do you use the beer lambert law to predict the absorbance at 405nm of a solution that is 11.0mM in the p-nitrophenolate ion, how do you show absorbance has no units

Homework Equations

A=ecl

The Attempt at a Solution

.650 = e x 1 x (74/2)??
dont think I am right at all...

 

[Ygggdrasil]

That's the correct way to setup the equation, but you should include units so that you can get the units of ε correct (ε is usually reported in units of L cm^-1 mol^-1).

 

[Blueshift5]

To calculate the molar extinction coefficient (ε) at 405nm using the Beer-Lambert Law, we can rearrange the equation to solve for ε:

ε = A/(cl)

Where A is the absorbance, c is the concentration in mol/L, and l is the path length in cm. In this case, we are given A = 0.650 and c = 74um (0.074mM), and assuming a path length of 1cm, we can plug these values into the equation to solve for ε:

ε = 0.650/(0.074 x 1) = 8.78 L/mol*cm

To predict the absorbance at 405nm for a solution that is 11.0mM in the p-nitrophenolate ion, we can use the same equation and solve for A:

A = εcl = (8.78 L/mol*cm) x (11.0mM) x (1 cm) = 0.097

Absorbance is a unitless quantity, as it is a measure of the amount of light absorbed by a substance, rather than a measure of the substance itself. The units of ε are L/mol*cm, which represent the molar absorptivity of a substance.