# A sample in a #1cm# cuvette gives an absorbance reading of #0.558#. If the absorptivity for this sample is #15000 L/(mol.cm)#, what is the molar concentration?

##### 1 Answer

The concentrations are (a)

#### Explanation:

**A. Molar concentration from absorbance**

To solve this problem, we use **Beer's Law**:

#color(blue)(bar(ul(|color(white)(a/a) A = epsilonlc color(white)(a/a)|)))" "#

where

We can rearrange this equation to get

#c = A/(epsilonl)#

In this problem,

Then,

**Molar concentration from transmittance**

Recall that **absorbance** is defined as

#color(blue)(bar(ul(|color(white)(a/a)A = log(I_0/I)color(white)(a/a)|)))" "#

where

**Transmittance**

#color(blue)(bar(ul(|color(white)(a/a)T = I/I_0color(white)(a/a)|)))" "#

∴

Also, **percent transmittance**

so

∴

#color(blue)(bar(ul(|color(white)(a/a)A = 2-log(%T)color(white)(a/a)|)))" "#

This last equation is **worth remembering** because it gives you an easy way to calculate absorbance from percent transmittance.

Now, we can use Beer's Law: