Question

Consider a buffer solution containing 0.100 M fluoride ions and 0.126 M hydrogen fluoride. What is the concentration of fluoride ions after the addition of 5.00 mL of 0.0100 M HCl to 25 mL of this solution?

Answer 1

To find the new concentration of fluoride ions in a buffer after adding HCl, we calculate the amount of HCl added in moles, which then reacts with fluoride ions, reducing their concentration in the solution. The new concentration of fluoride ions is determined by the remaining moles of fluoride divided by the new total volume of the solution.

Step-by-step explanation:

The student is working with a buffer solution and is interested in finding out the effect of adding a strong acid, specifically HCl, to this buffer. The key concept here is to recognize that the added HCl will react with the fluoride ions from the sodium fluoride part of the buffer to form hydrogen fluoride (HF). Since we know that 1 mole of HCl will react with 1 mole of fluoride ions (F-), we can calculate the change in concentration of fluoride ions due to the acid addition.

To start the calculation, we need to find the number of moles of HCl added:

• (5.00 mL) x (0.0100 M) = 0.05 mmol HCl

Since the solution volume after addition is:

• (25 mL buffer) + (5 mL HCl) = 30 mL

In the buffer reaction, the fluoride ions will react with the HCl to form HF, thus the concentration of fluoride ions will decrease by an amount equivalent to the moles of HCl added.

• (0.100 M F-) x (25 mL) = 2.5 mmol F-

• 2.5 mmol F - 0.05 mmol HCl = 2.450 mmol F- remaining

Finally, the new concentration of fluoride ions in the total volume is

• 2.450 mmol F- / 30 mL = 0.0817 M F-