What is the molarity of 5.0 L of a solution containing 200. g of dissolved CaCO3?

The molarity of a solution in which a particular compound is dissolved is defined as the number of moles of the compound dissolved per liter of the solution.

Here we have 5.0 L of solution which has 200 g of CaCO3 dissolved in it. The molar mass of CaCO3 is 100.0869 g/mol. This can be taken as 100 g/mol. approximately.

200 g of CaCO3 is 200/100 = 2 moles of CaCO3.

As the 2 moles of CaCO3 is dissolved in 5 L of the solution, the resulting molarity is given by 2/5 = 0.4 mol/liter.

What is the molarity of 5.0 L of a solution containing 20.0 g of dissolved CaCO3 (100g/mol)?

You are only allowed to ask one question at a time so I edited down accordingly.  We are looking at the molarity of a solution of 20 grams of CaCO3 dissolved in 5 L of water.  Molarity is defined as moles per liter, so we need to convert the mass of CaCO3 into moles of CaCO3.  We do this by dividing by the molecular weight of the compound (100 g/mol). 

20 g *(1 mole/100 grams) = 0.2 moles of CaCO3

Now we divide the number of moles by the total volume of the solution (5 L).

0.2 moles / 5 liters = 0.04 moles per liter

The the concentration of the solution is 0.04 M, or moles per liter.