Na has an atomic mass of 23.0g and O has an atomic mass of 16.0g. How many grams of Na are needed to completely react with 40.0g of O2?

The grams of Na that are needed to complete to react with 40 ... 0 g of O2 is calculated as below

find the moles of O2 used = mass/molar mass

= 40 g/32g/mol = 1.25 moles

write the reacting equation

4Na + O2 = 2Na2O

by use of mole ratio between Na to O2 which is 4 : 1

the moles of Na = 1.25 x 4 = 5 moles

mass of Na = mass x molar mass

= 5 moles x 23 g / mol = 115 moles