Suppose that you massed out 3.50 g of sodium carbonate, Na2CO3. How many moles would you have?

0.0330 moles Na2CO3

Explanation: You divide 3.50 grams by the molar mass of Na2CO3, which is 105.99 g/mol. The grams will cancel out and you're left with 0.0330 moles NA2CO3. The answer will have 3 sig figs.

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.