Question #3c9dd

The balanced chemical equation is essentially a description of what happens during a chemical reaction.

In this case, you have

#color(blue)(2)"H"_ (2(g)) + "O"_ (2(g)) -> color(purple)(2)"H"_ 2"O" _((l))#

The thing to remember about balanced chemical equations is that the stoichiometric coefficients can be used as mole ratios.

You can thus say that every #1# mole of oxygen gas that takes part in the reaction consumes #color(blue)(2)# moles of hydrogen gas and produces #color(purple)(2)# moles of water.

These mole ratios must always be true, regardless of the actual numbers of moles of each chemical species that takes part in the reaction.

So, you know that #color(blue)(2)# moles of hydrogen gas are needed to form #color(purple)(2)# moles of water. To convert this to grams, use the molar mass of hydrogen gas

#2 color(red)(cancel(color(black)("moles H"_2))) * overbrace("2.0159 g"/(1color(red)(cancel(color(black)("mole H"_2)))))^(color(darkorange)("molar mass of H"_2)) = color(darkgreen)(ul(color(black)("4.03 g")))#

I'll leave the answer rounded to three sig figs.