How many atoms are in a piece of paper? How many atoms are there in a common sheet of paper?
The paper is A4, i.e.  $210 \, \mathrm{mm} {\times} 297 \, \mathrm{mm}$ $\left(8.27 \, \mathrm{in} {\times} 11.7 \, \mathrm{in}\right).$
 A: You can estimate number of atoms by finding out average molar mass of paper and mass of one sheet of paper.
If we assume that paper is mainly composed of cellulose, we can neglect other components as insignificant. Then we find out that cellulose's molar mass is approximately $162.14 \, \mathrm{g}/\mathrm{mol}$. Mass of one sheet of paper is about $4.5 \, \mathrm{g}$.
Doing the math you get that one sheet of paper is approximately $0.02775 \, \mathrm{mol}$. With Avogadro's constant, we can calculate that there are$$
0.02775 \, \mathrm{mol} \times 6.02214 \cdot {10}^{23} \, \frac{\mathrm{molecule}}{\mathrm{mol}} ~=~ 1.07 \cdot {10}^{22} \, \mathrm{molecule}.$$
As in one cellulose molecule there are 21 atoms, the number of atoms is $2.247 \cdot {10}^{23}$ atoms.
A: So, a piece of paper is made of wood, and wood is some organic substance. I don't know what the chemical formula is, but let's say it's mostly carbon. In fact, let's just pretend it's all carbon, since you only want order of magnitude. Wikipedia tells me a piece of A4 paper weighs about five grams, and then I divide by the atomic mass of carbon and get $\approx 10^{23}$ atoms. This is on the order of Avogadro's number, which is generally what you get for small but macroscopic numbers of atoms.
