I'm unsure how to figure this question out.
"How many moles of Na2CO3 are there in 10.0 L of 2.0 M solution?"
Just remember the definition of molarity and rearrange the definition to suit what you have and what you want.
M = #moles/L soln.
Rearrange to obtain
# moles = M x L and that's easy enough to do.