The student conducts an experiment to determine the composition of a mixture of nahco3 and na2co3. the student places a sample of the mixture into a preweighted test tube that is attached to container that holds a drying agent. answer

When you talk about a drying agent, its purpose is to remove the water in the compound. The compound NaHCO₃ can be decomposed when heated. The reaction is:

2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

The drying agent could remove H₂O while CO₂ can escape as gas. Thus, you can determine the total amount of Na₂CO₃ in the test tube by weighing it. 

They are all full and don't need any more electrons in there outer shell.