Volumetric calculations help!

I really don't understand any of these questions, can anyone explain them?

1. 2.12g of anhydrous sodium carbonate were dissolved in water and the solution made up to 200cmз. 25.0cmз of the solution on titrating with nitric acid was found to neutralise 15.0cmз of the acid. What is the molarity of the acid?

2. A solution of Barium Hydroxide contains 17.1g dm-з. Calculate:

a) Its molarity

b) the volume of this barium hydroxide solution which would be required to react exactly with 25.0cmз of a solution containing 12.0g of ethanoic acid, CHзCOOH, per litre

3. 25.0cmз of a solution containing 1.86g of a metal carbonate, X₂COз, in 250cmз exactly reacted 27.0cmз of 0.100mol dm-з hydrochloric acid. Calculate:

a) the relative molecular mass of X₂COз

b) the atomic mass of X

I know this is a lot but can anyone explain it please??