Question

An aspirin tablet contains 0.325g of acetylsalicylic acid (C9H8O4).

How many acetylsalicylic acid molecules does it contain?

What is the total number of atoms in that sample?

Answer 1

(i) Find number of moleculs of $C_9H_8O_4$

$n = \frac{m}{M}$

$n(C_9H_8O_4) = \frac{0.325 g}{180.16 \frac{g}{mol}} = 0.00180 mol$

$N = N_A\times n$

$N(C_9H_8O_4) = 6.022\times 10^{23} mol^{-1} \times 0.00180 mol = 1.086\times10^{21} molecules$

(ii) find total number of atoms:

As 1 molecule of $C_9H_8O_4$ contains 9+8+4 = 21 atoms, then

$N_{total}(atoms) = 21\times N(C_9H_8O_4) = 21\times 1.086\times10^{21} = 2.281\times 10^{22} atoms$