Question

How many moles of ethane (C2H6) would be needed to react with 62.3 grams of oxygen gas?

Answer 1

Note that it says oxygen "gas"
So you need the atomic mass of oxygen gas

Look at your periodic table, you'll see 15.9994 under oxygen
Oxygen gas has a formula of O2 therefore,
(15.9994) times 2= Oxygen gas atomic mass=31.9988

Mol= Mass/Atomic Mass
=62.3 g/ 31.9988 g/mol = 1.95 mol

now look at the ratio of C2H6 and O2, notice there is an invisible number beside each of them, at that "invisible number" is =1

1 C2H6 + 1 O2 -> products

this means that for 1 mol of C2H6, 1 mol of O2 has to react with it

Thus as we have 1.95 moles of O2, we need 1.95 moles of C2H6