What mass of sodium fluoride must be added to 250. mL of a 0.100 M HF solution to give a buffer solution having a pH of 3.50 [K_a(HF) = 7.1* 10^-4]

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Quizplus · Accepted Answer

To find the mass of sodium fluoride (NaF) needed, we use the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the fluoride ion (F-) from NaF, and [HA] is the concentration of HF. Given pH = 3.50 and pKa = 3.14 (from the given Ka = 7.1 x 10^-4), we can solve for the ratio [A-]/[HA]. Rearranging the equation gives [A-]/[HA] = 10^(pH-pKa). Substituting the given values, we find [A-]/[HA] = 10^(3.50-3.14) ≈ 2.29. Since the initial concentration of HF is 0.100 M, and assuming complete dissociation of NaF, the moles of NaF needed equals 2.29 times the moles of HF. The volume of the solution is 250 mL or 0.250 L, so moles of HF = 0.100 M * 0.250 L = 0.025 moles. Therefore, moles of NaF = 2.29 * 0.025 moles = 0.05725 moles. The molar mass of NaF is approximately 42 g/mol, so the mass of NaF needed = 0.05725 moles * 42 g/mol ≈ 2.3 g.

What Mass of Sodium Fluoride Must Be Added to 250