Do 5 first please Answers to red problems are available in the Answer Appendix.

Question

Do 5 first please Answers to red problems are available in the Answer Appendix. 1. If suppose a chloride ion and a sodium ion are separated by a center-center distance of 5 A. Is the interaction energy (the energy required to pull them infinitely far apart) predicted to be larger if the medium between them is water, or if it is n-pentane? (See Table 2.5.) If Ca Nat, and F each have ionic radii 1.16 A. which ionic bond is stronger: Ca-F or Na-F? If Ca is often bound on the surface of a protein by carboxylic acid func- tional groups. If the pKa of a particular COOH group is 4.2, would you predict Ca to be most tightly bound at pH 8, pH 4.2, or pH 3? Explain your answer

Explanation / Answer

Answer (5)

(a)

Since NH4Cl is made by reacting a weak base (NH3) with a strong acid (HCl), the salt will produce a slightly acidic solution when dissolved in water. That is, the NH4+ ion will hydrolyze to produce equally small amounts of H3O+ and NH3.

Molarity . . . . .NH4+ + H2O <==> H3O+ + NH3
Initial . . . . . . . . 1 . . . . . . . . . . . . . .0 . . . . . .0
Change . . . . . .-x . . . . . . . . . . . . . . .x . . . . . .x
At Equil. . . . .1 - x . . . . . . . . . . . . . .x . . . . . .x

The Kb for NH3 = 1.8 x 10^-5. Ka for NH4+ ion = (Kw / Kb) = (1 x 10^-14 / 1.8 x 10^-5) = 5.6 x 10^-10

Ka = [H3O+][NH3] / [NH4+]

= x^2 / (1 - x) = 5.6 x 10^-10
x = 2.4 x 10^-5 M = [H3O+]
pH = -log[H3O+] = -log (2.4 x 10^-5 ) = 4.6

(b)

Here 10mL of 1.0M NaOH will react with the NH4Cl to produce 10mL of 1.0M NH4OH - there is 30m L 1.0M NH4Cl remaining
pOH = pKb + log ([salt]/[base])
pOH = 4.74 + log ( 30/10)
pOH = 4.74 + log 3
pOH = 4.74 + 0.48
pOH = 5.22
pH = 14.00-pOH
pH = 14.00 - 5.22
pH = 8.78

(c)

the 30 mL of 1.0M NaOH will react with the NH4Cl to produce 30mL of 1.0M NH4OH - there is 10m L 1.0M NH4Cl remaining
pOH = pKb + log ([salt]/[base])
pOH = 4.74 + log ( 10/30)
pOH = 4.74 + log (0.33)
pOH = 4.74 - 0.4771
pOH = 4.26
pH = 14.00-pOH
pH = 14.00 - 4.26
pH = 9.73

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.