# Now you try it...

John and Suzy want to measure the strength of various mixtures of red koolaid. After finding the proper wavelength, they run 10, 20, 40, and 80% solutions through a spec, and get the following data:

concentration

10%

20%

40%

80%

absorbance

.31 OD

.59 OD

1.2 OD

2.3 OD

Then they go to the State Fair and get samples of red koolaid from 3 different vendors. These samples have absorbances of 0.7, 1.5, and 2.6 OD. How strong is each glass of koolaid?

### First, what is “e”?

(To make this problem interactive, turn on javascript!)

• I need a hint ... : Remember that “e” is the rate at which stuff in the water absorbs light.

• ...another hint ... : Slope = rate, so find the slope of the graph

• ...another hint ... : How much does the absorbance change as you go from 0 to 10% koolaid?

• ...another hint ... : Since Beer's Law says the relationship is DIRECTLY PROPORTIONAL, then the absorbance will change 1/10 th as much from 0 to 1% as from 0 to 10%.

#### I think I have the answer: e = 0.03 OD / percent koolaid.

So far, we know that e = 0.03 OD / percent koolaid. Fitting that into our equation, we get

OD = 0.03 c

### Now that you have “e”, what are the concentrations of the three koolaid samples (their OD's were 0.7, 1.5, and 2.6 AU's)

(To make this problem interactive, turn on javascript!)

• I need a hint ... : you can write an equation for concentration based on absorbance.

• ...another hint ... : To get the equation for concentration, solve for c...

• ...another hint ... : Solving for c...

OD = e c
c = OD / e
c = OD / 0.03

• ...another hint ... : Use Google as a calculator!