To solve this problem, we use Beer's Law: A= ε.l.c
A is the absorbance- 0,558
ε is the molar absorptivity- is 15000 L⋅mol-1cm-1
l is the length of the cuvette- 1 cm
c is the molar concentration
Applying the formula,
0,558= 15000 x 1 x c
0,558/15000= c
c= 3.72×10⁻⁵ mol⋅L⁻¹
