External Standardization for Copper

To determine the concentration of Cu²⁺ in a sample of water obtained from a local pond, the following solutions were prepared:

Solution I: 5.00 mL of sample diluted to 10 mL in a volumetric flask

Solution II: 5.00 mL of sample + 20 μL of 100.0 ppm Cu²⁺ diluted to 10 mL in a volumetric flask

(a) Analysis of solution I gives a signal of 0.215 (arbitrary units) and an analysis of solution II gives a signal of 0.441.  What is the concentration of Cu²⁺ in the sample?

(b) Suppose that an external standardization shows that the relationship between ppm Cu²⁺ and the signal isn’t linear.  Is a one-point standard addition still possible?  Be sure to clearly justify your answer.