Problem summary

2CO(g)		+	O2(g)		2CO2(g)
amount (n)	4.0 mol	reacts with	2.0 mol	to give	4.0 mol
molar mass (M)	28.0 g mol–1		32.0 g mol–1		44.0 g mol–1
mass (m)	112 g	reacts  with	64.0 g	to give	176 g

You should have learned from this problem that
 

1. the relative amounts in moles of substances consumed or produced by a reaction depends on their coefficients in the balanced equation for the reaction.
    
   Note that the ratio of the amounts in moles in the second line of the table is the same as the ratio of the coefficients in the balanced equation.
    
2. the mass in grams required (consumed) or produced can be calculated from the amount in moles (n) and the molar mass (M)* of the substance (sum of the molar masses of the atoms).
    
   m = nM
3. mass is conserved in chemical reactions because the total mass of the reactants equals the total mass of the products.