4Na(s) + O2(g) ----> 2Na2(little2)O(s): help with mass calculation in reaction?

I get different answers everytime I try this one, none matching the correct answer in the book.

Sodium reacts with oxygen to produce sodium oxide. If you have 18.0 grams Na, how many grams O2 are required for reaction?

To the equality pasted in the question line.

Please show how to get the answer because I am confused with the coefficients.

Answer:
If you look at molar ratios (the ratio of coefficients), you can figure this one out.

Sodium has a coefficient of 4, and you want to compare it to the coefficient of O2(which is 1) to figure out how many grams of O2 you need.

First, convert 18 g Na into moles using the atomic mass of Na: (18 g Na) / (22.99 g/mol) = .783 moles

Now, using the molar ratios I talked about, find moles of O2:
(.783 moles Na) * (1 mole O2 / 4 moles Na)= .196 moles O2

Now you can easily convert from moles to grams using the formula weight of O2:
(.196 moles O2) * (32 g/mol) = 6.26 grams O2
no. of moles of Na = 18/23.1 = 0.779 mol

no. of moles of O2 required = 0.25 x 0.779 = 0.195 mol

Mass of O2 = 0.195 x (2 X 16) = 6.23 g

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