Question

36 grams of aspirin is burnt with a flame in presence of excess oxygen until nothing remains. The byproduct gases are collected and it is found that there are 79.2 g of pure CO2 as one of the products. How many carbon atoms are there in each formula unit of Aspirin (MW = 180g)?

Answer 1

Mass of $CO_2=79.2g$

Therefore number of moles $=\frac{mass}{molar mass}= \frac{79.2g}{44g/mol}$

$=1.8mole CO_2$

1 mole $CO_2$ contains only 1 mole C

Therefore 1.8 $CO_2$ must contain 1.8 mole C

Mass of aspirin$=36g$

Molar mass$=180g/mol$

Therefore moles of aspirin$=\frac{36}{180} =0.2moles$

Since total mole of C in 0.2 mole of aspirin = 1.8 mole this means in 1 mole aspirin there will be $\frac{1.8}{0.2}$ $=9$

Hence aspirin must contains 9 C atoms