Question

In the following reaction, how many grams of oxygen will react with 10.47 grams of benzene (C6H6)?
2C6H6 + 15O2 12CO2 + 6H2O
The molar mass of benzene is 78.1074 grams and that of O2 is 32 grams.

Answer 1

First work out the moles of benzene using: mass/molar mass

10.47g/78.1074gmol-1 = 0.134mol (3 significant figures)

You then deduce the moles. If 2 moles of benzene is used to make 15 moles of O2 you need to make that number into 15 moles. Right now that is the amount of moles for 2 moles of an element.
To get the number for 15 moles, divide the answer we got by 2, then times it by 15.
0.134/2 = 0.067 and then 0.067 x 15 = 1.005mol

This means oxygen is 1.005 moles. From this we can work out the mass in g by using this: molar mass x moles.

32gmol-1 x 1.005mol = 32.16g

Therefore the answer is 32.16g.
I hope this is right, and I hope it helps you in the future :)