Question

What is the mass, in grams, of 7.20×1020 molecules of caffeine, c8h10n4o2?

Answer 1

To determine the mass, you need to know the molecular weight of the c8h10n4o2 . The molecular weight of c8h10n4o2 would be: 8*12 + 10*1 + 4*14 + 2*16= 194g/mol.
To convert the number of molecules into moles, you need to divide it with 6.02 * 10^23. The calculation of the mass of c8h10n4o2 would be: 
(7.20×10^20 molecules)  /(6.02 * 10^23 molecule/mol)  * 194g/mol= 232 * 10^-3 grams= 0.232 grams

Answer 2

Answer: 0.23 grams

Explanation:

According to avogadro's law, 1 mole of every substance weighs equal to its molecular mass and contains avogadro's number $6.023\times 10^{23}$ of particles.

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given molecules}}{\text{Avogadro's number}}=\frac{7.20\times 10^{20}}{6.023\times 10^{23}}=1.2\times 10^{-3}moles$

1 mole of caffeine $(C_8H_{10}N_4O_2)$ weighs = 194 g

$1.2\times 10^{-3}$  moles of  $(C_8H_{10}N_4O_2)$ weigh = $\frac{194}{1}\times 1.2\times 10^{-3}=0.23g$

Thus mass of caffeine $(C_8H_{10}N_4O_2)$ is 0.23 grams.