When is it preferable to use a plastic volumetric flask instead of a more accurate glass flask?

solution;

Typically plastic volumetric flasks are used when performing trace analysis and analyses may be absorbed OR desorbed from the glass.  

An examples is estimating [Na+] in aqueous solutions by a a process like ion-chromatography--if you acid wash your glass volumetric flasks then Na+ in the solution can be absorbed by the glass and then if you base wash your glass volumetric flask, then you may have excess Na+ leaching out into your solution. So to avoid leaching out or absorption of Na+ ions, you would want to use plastic volumetric flasks.  

The other instance you would use plastic vessels is measuring out solutions of strong bases like KOH and NaOH--glass slowly dissolves in solutions of bases--you will get potassium or sodium silicate in the end and glass is permanently etched by strong bases--so in these instances you would want to use plastic vessels  

As regards acids--the only problem you would have is with HF--glass would be etched/dissolved to form H₂SiF₆ and related soluble compounds  

conduction is the transfer of thermal energy (heat in transfer) due to collisions between the molecules in the object. collisions between adjacent atoms and molecules transfer kinetic energy from the warmer to the cooler object. the objects must be in physical contact.

answer; the numbers of valence electrons in the acetic acid molecule (ch3co2h) is 24;