Final answer:
The ratio of NaF to HF to create a buffer with pH = 4.00 involves using the Henderson-Hasselbalch equation and knowledge of the pKa for HF, then calculating the mass of NaF needed from the determined ratio.
Step-by-step explanation:
The question focuses on determining the correct ratio of NaF to HF to create a buffer solution with a specific pH value. According to the Henderson-Hasselbalch equation, the ratio of the concentration of the salt (conjugate base) to the acid (HF) is crucial for setting the desired pH level of a buffer.
Given that the buffer needs a pH of 4.00, and the pKa of HF is 3.75, the required ratio can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). Rearranging the equation and solving for the ratio [A-]/[HA], it would fall between 1 and 10, affirming that the pH of the buffer will lie between pKa and pKa+1, or between 3.75 and 4.75.
To calculate the exact amount of NaF needed when starting with 100 ml of 1 M HF to achieve a pH of 4.00, one can determine the ratio and then convert it into grams of NaF required using the known molar mass of NaF which is 41.99 g/mol.