Calcium nitrate dissolves as follows: . how many moles of ca(no 3) 2 are required to make 2.0 liters of solution in which the no 3 - concentration is 0.20 m?

If the volume of the solution is 2.0 liters and the concentration of NO3 - is 0.20 m, the number of mol of NO3- in the solution would be:
mol= volume * concentration= 2 liters * 0.2 mol/liter= 0.4 mol

The equation of ion formed by ca(no 3) 2 would be:
Ca(NO3)2 ==> 1 Ca + 2 NO3
For every 1 mol of Ca(NO3)2 will produce 2 mol of NO3-. So, the amount of Ca(NO3)2 needed to make 0.4 mol of NO3- would be: 0.4 mol / (2/1)= 0.2 mol