Question

Which of the following would form a buffer if added to \(295 \mathrm{~mL}\) of \(0.380 \mathrm{M} \mathrm{HCl}\) ? (a) \(0.100 \mathrm{~mol}\) of \(\mathrm{NH}_{4} \mathrm{Cl}\) (b) \(0.033 \mathrm{~mol}\) of \(\mathrm{KF}\) (c) \(0.167 \mathrm{~mol}\) of \(\mathrm{Sr}(\mathrm{OH})_{2}\) (d) \(0.279 \mathrm{~mol}\) of \(\mathrm{KNO}_{2}\) (e) \(0.112 \mathrm{~mol}\) of \(\mathrm{KClO}\)

Short answer

Answer: 0.279 mol of KNO2

Step-by-step solution

Determine the concentrations of HCl before adding any substances

: Before adding anything, we have a 0.380 M HCl solution. So to start, calculate the moles of HCl in the initial solution: Moles of HCl = (Volume)(Concentration) = (295 mL)(0.380 mol/L) = 0.295 L × 0.380 mol/L = 0.112 mol

Analyze each substance and whether it would create a buffer solution

: (a) 0.100 mol of NH4Cl: Here, we have NH4Cl, which is a salt of a weak base (NH4+) and a strong acid (Cl-). When NH4Cl is added, the weak base NH4+ will react with the strong acid HCl. Since there is excess HCl, there will be no conjugate pair formed. Hence it won't form a buffer solution. (b) 0.033 mol of KF: KF is a salt of a weak acid (HF) and a strong base (K+). Since the weak acid is not present to react with HCl, it won't form a buffer. (c) 0.167 mol of Sr(OH)2: Sr(OH)2 is a strong base, and it reacts completely with the acid HCl. There won't be any weak base left after the reaction, so it won't form a buffer solution. (d) 0.279 mol of KNO2: KNO2 is a salt of weak acid (HNO2) and strong base (K+). It dissociates into K+ and NO2- ions. The NO2- ion, a conjugate base, reacts with HCl to produce weak acid HNO2. Since the amount of NO2- (0.279 mol) is more than HCl (0.112 mol), we'll still have NO2- ions after the reaction. Hence, we have a weak acid (HNO2) and its conjugate base (NO2-) existing simultaneously, and it forms a buffer solution. (e) 0.112 mol of KClO: KClO is a salt of a strong acid (K+) and a weak base (ClO-). Due to the absence of a suitable conjugate acid pair, it cannot react with HCl to form a buffer solution.

Choose the correct option

: Based on the analysis, the correct option is (d) 0.279 mol of KNO2 as it forms a buffer solution with HCl by providing a weak acid (HNO2) and its conjugate base (NO2-) simultaneously in the resulting solution.