Hey there!
Molar mass:
H2O2 = 34,01 g/mol
H2O = 18,01 g/mol
By the stoichiometry of the reaction:
2 H2O2 = 2 H2O + O2
2 * 34,01 g H2O2 -------- 2 * 18,01 g H2O
42,0 g H2O2 ------------ m
m = ( 42,0 * 2 * 18,01 ) / ( 2 * 34,01 )
m = 1512,84 / 68,02
m = 22,24 g H2O
Therefore:
1 mole H2O ------------ 18,01 g
?? moles H2O ---------- 22,24 g
moles H2O = 22,24 * 1 / 18,01
moles H2O = 22,24 / 18,01
moles H2O = 1,23 moles
Hope this helps!
