An artificial fruit beverage contains of tartaric acid, , and of its salt, potassium hydrogen tartra — Ionic Equilibrium Chemistry Question
Question
An artificial fruit beverage contains $30.0\text{ g}$ of tartaric acid, $\text{H}_2\text{C}_4\text{H}_4\text{O}_6$, and $18.8\text{ g}$ of its salt, potassium hydrogen tartrate, per litre. What is pH of the beverage? For tartaric acid, $K_{a1} = 5.0 \times 10^{-4}$, $K_{a2} = 4 \times 10^{-9}$.
💡 Solution & Explanation
Molar mass of tartaric acid ($\text{H}_2\text{A}$) = $150\text{ g/mol}$. Moles of $\text{H}_2\text{A} = 30.0 / 150 = 0.20\text{ M}$. Molar mass of potassium hydrogen tartrate ($\text{KHA}$) = $188\text{ g/mol}$. Moles of $\text{HA}^- = 18.8 / 188 = 0.10\text{ M}$. This creates an acidic buffer. $\text{pH} = pK_{a1} + \log([\text{HA}^-]/[\text{H}_2\text{A}]) = -\log(5.0 \times 10^{-4}) + \log(0.10/0.20) = (4 - 0.7) + \log(0.5) = 3.3 - 0.3 = 3.0$. Therefore, correct answer is 3.