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1) write and balance the equation of calcium carbonate plus sulfuric acid that produce calcium sulfate plus carbonic acid.
2) calculate the moles of CaSO2 in 50kg of CaSO4 and convert tho the 100% by multiplying by the degree of conversion (0.85).
3) you will find that the stoichimetry of the reaction is 1:1 then the same amount of moles of CaSO4 will be necessary of sulfuric acid.
4) calculate the grams of sulfuric acid that will contain that number of moles. m= n/MM
The molecular mass (MM) of H2SO4= 98.1 g/mol
5) I assume that the 20% sulfuric acid is 20 g/100ml (w/v). Or you can assume that the 20% is 20 g/100g (w/w). In the first assumption you will get your answer in volume (mL), in the second you will get the answer in mass (g). To convert the mass to volume you will need to know the density that is not provided by the problem.
Multiply the g from step 4 by 5=100mL/20g and you will have the answer in mL, or,
Multiply the g from step 4 by 5=100g/20g and you will have the answer in g.
Note: the 78% of CaCO3 is an additional information not needed to solve the problem at least they ask you how much CaCO3 you will need