Copper was determined in a river water sample by atomic absorption spectrometry and the method of standard additions. For the addition, 100.0 μL of a 1000.0-μg/mL Cu standard was added to 100.0 mL of solution. The following data were obtained:
Absorbance of reagent blank = 0.020
Absorbance of sample = 0.520
Absorbance of sample plus addition - blank = 1.020

(a) Calculate the copper concentration in the sample.
(b) Later studies showed that the reagent blank used to obtain the preceding data was inadequate and that the actual blank absorbance was 0.100. Find the copper concentration with the appropriate blank, and determine the error caused by using an improper blank.