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Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 12
13-25. To obtain the absorptivity of dichromate, we must first convert the concentration of Cr in 
μg/mL to the concentration of 2
2 7Cr O − in g/L. We write for x μg Cr/mL, 
2
2 7Cr O
 g Cr
, g/L = 
x
c −
μ
mL
610 g Cr−
×
g Crμ
310 mL
×
2
2 7
2
2 7
215.988 g Cr O
L mol Cr O
−
−
×
mol Cr
×
51.9961 g Cr
2
2 71 mol Cr O −
×
2 mol Cr
0.215998 = 
2 51.9961
x
×
 
A plot of absorbance vs. the concentration of 2
2 7Cr O − in g/L will give the absorptivity in 
L g–1 cm–1 as the slope. To convert this to the molar absorptivity, we multiply by the 
molecular mass of , 215.988 g mol2
2 7Cr O − –1.