**Balanced equation**

#"2H"_2 + "O"_2"##rarr##"2H"_2"O"#

#color(red)(1.# Calculate mol #"H"_2"# by dividing its given mass by its molar mass #("2 g/mol")#. Do this by multiplying by the inverse of the molar mass.

#color(blue)(2.# Calculate mol #"H"_2"O"# by multiplying mol #"H"_2"# by the mol ratio between #"H"_2"O"# and #"H"_2"# from the balanced equation, with #"H"_2"O"# in the numerator.

#color(green)(3.# Calculate mass #"H"_2"O"# by multiplying by the molar mass of #"H"_2"O"# #("18 g H"_2"O")#.

#color(red)8color(black)cancel(color(red)("g H"_2))xx(color(red)(1color(black)cancel(color(red)("mol H"_2))))/(color(black)cancel(color(red)(2"g H"_2)))xx(color(blue)2color(black)cancel(color(blue)("mol H"_2"O")))/(color(blue)2color(black)cancel(color(blue)("mol H"_2)))xx(color(green)18color(green)("g H"_2"O"))/(color(green)1color(black)cancel(color(green)("mol H"_2"O")))="70 g H"_2"O"# (rounded to one significant figure)