Question

How many grams of `O_2(g)` are needed to completely burn `21.4` `g` of `C_3H_8(g)` in the reaction `C_3H_8(g) + 5O_2(g) -> 3CO_2(g) + 4H_2O(g)`?

Answer 1

Find the number of moles of `C_3H_8`: `n=m/M=21.4/44=0.49` `mol`. Each mole of `C_3H_8` requires `5` `mol` of `O_2`, so we need `5xx0.49=2.45` `mol`. The mass will be `m=nM=2.45xx32=78.4` `g`.