If 3.00 moles of #H_2O# are produced, how many grams of hydrogen gas are used?

If the equation is
#H^+# + #O^-2# #=># #H_2O#

and if you balance it,
#4H^+# + #2O^-2# #=># #2H_2O#

Assume that:
n = number of moles
m = mass of substance
M = molar mass (equivalent to atomic weight on the periodic table)

#n = m -: M#

The mole ratio between #H : H_2O# is #4:2#. If you simplify the mole ratio into simplest form, it becomes #2:1#

So, if 1 mole of #H_2O# gives you 2 moles of #H^+#,

then 3.00 moles of #H_2O# must give you:
#[3.00 mol xx 2]# = 6.00 moles of #H^+#

You now know that Hydrogen gas has 6.00 moles. (n)

Your next step is to find the molar mass (M) of hydrogen (#H^+#). If you look into your periodic table, the molar mass of (#H^+#) is 1.00 g/mol.

Since you have found your number of moles (n) and molar mass (M) for hydrogen, your final step is to determine the mass (m).

Looking back at the #n = m-: M# formula, to find mass of substance (m), you need to flip it and it becomes #m = M xx n#

The mass of hydrogen gas is:
m = 1.00 g/mol #xx# 6.00 moles = 6.00 grams of #H^+# gas.

Therefore 6.00 grams of hydrogen gas is being used.