Question

Aspirin is well known as a pain reliever (analgesic) and as a fever reducer (antipyretic). It has a molar mass of 180.2 g/mol and a composition of C 60.0%, H 4.5% and O 35.5%. Calculate the molecular formula of aspirin.

Answer 1

Answer: The molecular formula of aspirin is $C_8H_8O_2$

Explanation:

If percentage are given then we are taking total mass is 100 grams.

So, the mass of each element is equal to the percentage given.

Mass of C= 60.0 g

Mass of H = 4.5 g

Mass of O = 35.5 g

Step 1 : convert given masses into moles.

Moles of C =$\frac{\text{ given mass of C}}{\text{ molar mass of C}}= \frac{60.0g}{12g/mole}=5.00moles$

Moles of H =$\frac{\text{ given mass of H}}{\text{ molar mass of H}}= \frac{4.5g}{1g/mole}=4.5moles$

Moles of O =$\frac{\text{ given mass of O}}{\text{ molar mass of O}}= \frac{35.5g}{16g/mole}=2.22moles$

Step 2 : For the mole ratio, divide each value of moles by the smallest number of moles calculated.

For C = $\frac{5.00}{2.22}=2$

For H = $\frac{4.5}{2.22}=2$

For O =$\frac{2.22}{2.22}=1$

The ratio of C : H: O= 2: 2: 1

Hence the empirical formula is $C_2H_2O$

The empirical weight of $C_2H_2O$ = 2(12)+2(1)+1(16)= 42g.

The molecular weight = 180.2 g/mole

Now we have to calculate the molecular formula.

$n=\frac{\text{Molecular weight }}{\text{Equivalent weight}}=\frac{180.2}{42}=4$

The molecular formula will be=$4\times C_2H_2O=C_8H_8O_2$

Answer 2

Answer:

C9H8O4

Explanation:

60/12=5   4.5/1= 4.5   35.5/16= 2.25

5/2.25=2.22  4.5/2.25=2  2.25/2.25=1

2.22x4=9C  2x4=9H   1x4=4O

h 1x4=4