Question

Rank the following species in order of increasing acidity. Explain your reasons for ordering them as you do.

Short answer

${\text{K}}_{\text{a}}$ is known as the acid dissociation constant and its value indicates the relative strength of the acid. is defined as the negative logarithm (base ) of . Mathematically, . value of strong acids is usually zero or even negative while the value of weaker acids are greater than .

Step-by-step solution

Ka and pKa

${\text{K}}_{\text{a}}$ is known as the acid dissociation constant and its value indicates the relative strength of the acid. is defined as the negative logarithm (base ) of . Mathematically, . value of strong acids is usually zero or even negative while the value of weaker acids are greater than .

Species in order of increasing acidity

The strength of an acid or base is given by itsvalue. The smallervalue means stronger is the acid while the largervalue means stronger is the base.

$\begin{array}{l}{\text{pK}}_{\text{a}}\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}HF\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}3.87}\\ {\text{pK}}_{\text{a}}\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}HCN\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}9.23}\\ {\text{pK}}_{\text{a}}{\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}NH}}_{\text{3}}\text{\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}36}\\ {\text{pK}}_{\text{a}}{\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}H}}_{\text{2}}{\text{SO}}_{\text{4}}\text{\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}-2.8}\\ {\text{pK}}_{\text{a}}\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}HCOOH\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}3.76}\\ {\text{pK}}_{\text{a}}{\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}CH}}_{\text{4}}\text{\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}50}\\ {\text{pK}}_{\text{a}}{\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}H}}_{\text{2}}\text{O\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}15.7}\end{array}$