дано
m(Ba(OH)2) = 1.71 g
------------------------
m(BaSO4)-?
Ba(OH)2+H2SO4-->BaSO4+2H2O
M(Ba(OH)2) = 171 g/mol
n(Ba(OH)2) = m/M = 1.71 / 171 = 0.1 mol
n(Ba(OH)2) = n(BaSO4) = 0.1 mol
M(BaSO4) = 233 g/mol
m(BaSO4) = n*M = 0.1 * 233 = 2.33 g
ответ 2.33 г