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.