A solution contains 0.0180 M HNO3, 0.0268 M HI, and 0.250 M formic acid, HCOOH. What is the pH?
To determine the pH of the solution, we need to calculate the concentration of H+ ions in the solution, as pH is a measure of the concentration of H+ ions in a solution.
Given that the solution contains three different acids, HNO3, HI, and formic acid (HCOOH), we need to consider their contribution to the concentration of H+ ions.
Let's start by calculating the concentration of H+ ions from each acid:
1. HNO3:
HNO3 is a strong acid, which means it dissociates completely in water, producing H+ ions. Therefore, the concentration of H+ ions from HNO3 is equal to the concentration of HNO3 itself, which is 0.0180 M.
2. HI:
Similar to HNO3, HI is also a strong acid that dissociates completely in water. Thus, the concentration of H+ ions from HI is equal to the concentration of HI itself, which is 0.0268 M.
3. Formic acid (HCOOH):
Formic acid is a weak acid, and therefore, it undergoes partial dissociation in water. To calculate the concentration of H+ ions from formic acid, we need to consider its dissociation equilibrium. The dissociation equation for formic acid can be written as follows:
HCOOH ⇌ H+ + HCOO-
The acid dissociation constant for formic acid, Ka, is typically given as 1.8 × 10^-4 at 25°C. Using this Ka value, we can set up an equilibrium expression:
Ka = [H+][HCOO-] / [HCOOH]
Since the concentration of H+ and HCOO- ions is assumed to be the same (x) due to a 1:1 ratio, the concentration of HCOOH will decrease by x.
Thus, the equilibrium expression becomes:
Ka = (x * x) / (0.250 - x)
Since the equation is quadratic, we need to solve it to find the concentration of H+ ions (x):
1.8 × 10^-4 = (x^2) / (0.250 - x)
Solving this quadratic equation will give us the concentration of H+ ions from formic acid.
Once we have calculated the concentrations of H+ ions from each acid, we can add them together to get the total concentration of H+ ions in the solution. Finally, we can use the concentration of H+ ions to determine the pH using the pH formula:
pH = -log[H+]
Plug in the concentration of H+ ions into the formula to obtain the pH value.