To find the mass of water, you need to (1) convert moles HNO₃ to moles H₂O (via mole-to-mole ratio from reaction coefficients) and then (2) convert moles H₂O to grams H₂O (via molar mass).
The molar mass of water can be found by combining the element's molar masses times their quantities. The ratios/conversions should be arranged in a way that allows for the cancellation of units (desired unit should be in the numerator).
Molar Mass (H₂O): 2(1 g/mol) + 16 g/mol
Molar Mass (H₂O): 18 g/mol
S + 6 HNO₃ ---> H₂SO₄ + 6 NO₂ + 2 H₂O ^ ^
19.7 moles HNO₃ 2 moles H₂O 18 g -------------------------- x ----------------------- x --------------------- = 118.2 g H₂O 6 moles HNO₃ 1 mole H₂O
