What is the mass of 2.5 moles of na2o?
its a little 2 on the equation

I'll be using dimensional analysis to help out.

2.5mol Na2O * ( X gNa2O / 1 molNa2O) = ? gNa2O 

X gNa2O is the molar mass of the molecule. To find that we must add up the molar masses of all of the elements in the molecule. 

There are 2 Na (Sodium). 
And 1 O (Oxygen). 

The Molar Mass of Na = 2 * 22.99 g/mol (It's multiplied by 2 because there are 2 of them in the molecule)

The Molar mass of O = 16.00 g/mol

If you are confused on where I found them. Let me enlighten you! I found them from a periodic table of elements. 

Add those two values together to get a total molecular molar mass of: 
38.99g/mol 

2.5mol Na2O * ( 38.99 gNa2O / 1 molNa2O) = 97 gNa2O