The student uses 25.0cm3 of 1.60mol/dm3 sodium hydroxide to prepare the crystals.
2naoh(aq) +h2so4+8h2o-> na2so4 10h2o

calculate the maximum mass of hydrated sodium sulfate crystals that can be formed

First you have to calculate how many mols of NaOH are in the 25.0 cm3. As one dm3 is equal to 10cm*10cm*10cm=1000 cm3, there are 0.0016 mol/cm3 and therefore 25cm3*0.0016 mol/cm3=0.04 mol NaOH. 
According to the stochiometrics of the reaction a maximum yield of 0.5*0.04=0.02 mol Na2SO4 can be formed. Multiplying by the molar mass gives the mass: 0.02mol*142.04g/mol=2.84 g 

The number of protons in the nucleus of the atom is equal to the atomic number . The number of electrons in a neutral atom is equal to the number of protons. The mass number of the atom is equal to the sum of the number of protons and neutrons in the nucleus.