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 

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