Astudent weights out 635.94 g na2co3 and dissolved it in 2.0 l water. what is the molarity of his solution?

You have to find out the number of moles of the substance.

I will use rounded numbers for the atomic weights because every periodic table is different.

2Ca = 2 * 40 =  80

1 C   = 1 * 12  =   12

3O = 3  * 16  =   48

Atomic Mass = 140

moles = given mass / molar mass

moles = 635.94 / 140

moles = 4.54

Now you need to find the concentration

C = moles / Volume in liters.

C = 4.54/ 2.0 L

C = 2.27 moles / L

Technically since the volume is only 2 significant digits, your answer should be 2.3 moles / L

If you do not have this as a choice, what are your choices.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.

answer; 4 electrons;