Which of the two : O₂NCH₂CH₂O–  or CH₃CH₂O– is expected to be more stable and why ? 

O₂N← CH₂← CH₂← O^–is more stable than O₂N← CH₂← CH₂ ← O^– because NO₂ group has -I- effect and hence it tends to disperse the –Ve charge on the O-atom. In constrast, CH₃CH₂  has +I-effect. It, therefore, tends to intensify the –Ve charge and hence destabilizes it.