Well for the first question, I think there is a problem with the temperature. The temperature should be in Kalvin, therefore it should be 298 (273+25)K not 275K. Check the answer after you plug in the right temperature. If this doesn't do the trick, you should check the volume of the solution. It says 1.00 dm^3 which is 100L.
For the second question, you should consider the number of particles existing in the solution. For example, when 1m of NaCl is dissovled in water, the decrease of freezing point is twice of 1m of molecules that do not ionize such as sugar. In this case, since the compound is an acid you should google it and find how many particles will exist due to the ionization.