Question

Acetyl salicylic acid $(mol.wt.=180)$ called aspirin is a pain killer with $p{K}_a=6$. It two tablets each of $0.09gm$ mass containing aspirin are dissolved in $100mL$ solution. Its pH will be:

• A. $0.5$
• B. $1.0$
• C. $4.0$
• D. $2.0$

Answer 1

The correct option is D $2.0$

Molecular weight of Aspirin= $180.157g/mol$

(Refer to Image 1), So it is a monoprotic acid.

Now, $0.09gm$ of aspirin= $\frac{0.09}{180.157}=0.0005$ moles

$\Rightarrow pH=\frac{1}{2}p{K}_a-\frac{1}{2}\log C=\frac{1}{2}\times 2-\frac{1}{2}\log (0.0005\times 10)$

$=1+1.15=2.15\approx 2$ .