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.

? you expect us to answer this? ? this is your schoolwork, i don't think we can with this, i think it would considering being