Question

Formic acid HCOOH (Ka = 1.8 × 10 ^ -4) is the irritant associated with ant bites. Calculate the PH and the percentage of an aqueous solution containing 75 g of HCOOH per liter of solution.

Answer 1

First, calculate the concentration of hydronium.

`K_a=\frac{\lbrack H_3O^+\rbrack\lbrack\text{HCOO}^-\text{\rbrack}}{\lbrack\text{HCOOH\rbrack}}`

Where the concentration of HCOOH is 75 g/L, or 75 M.

`\begin{gathered} K_a=(x\cdot x)/(75M)=1.8*10^(-4) \\ x^2=135*10^(-4) \\ x=\sqrt[]{135*10^(-4)} \\ x\approx0.116 \end{gathered}`

Therefore, the concentration of hydronium is 0.116 M.

Then, we find the pH.

`pH=-\log \lbrack H_3O^+\rbrack`
`pH=-\log \lbrack0.116\rbrack\approx0.94`

Therefore, the pH is 0.94, which means is a highly acidic solution.