how many grams of sodium metal are needed to give 781 g of hydrogen?

When reacted with H2O?

2Na + 2H2O ==> 2NaOH + H2
mols H2 you want = 781g/molar mass H2.
Using the coefficients in the balanced erquation, convert mols H2 you want to mols Na to start.
Then convert mols Na to g. grams = mols x atomic mass Na.