Question

17. In a compound, oxygen generally has an oxidation number (state) of A) +2 B) +1 C) 0 D) -2 18. Acetic acid has a pk, of 4.75. A solution is made to be 0.35 M in acetic acid and 0.20 Min sodium acetate (the sodium salt of the conjugate base). The pH is: A) 4.80 B) 2.81 C) 4.51 D) 4.99 E) 7.00 19. A process has a negative enthalpy change and a positive entropy change. The process is spontaneous at A) no temperature B) all temperatures C) low temperatures D) high temperature 20. 25.87 mL of aqueous HCl is titrated to endpoint with 28.16 mL of aqueous NaOH. The NaOH concentration is 0.1146 M. What is the HCl concentration? A) 0.1146 M B) 0.2292 M C) 0.1247 M D) 0.0624 M

Answer 1

17. Oxygen has 6 electrons in its outermost shell. It generally accepts 2 electrons to complete it's octet. The oxidation state of oxygen is usually -2 except in peroxides (H2O2) where it is -1, and in compounds with fluroine (OF2) where it is +2.

Hence, Option-D i.e, -2 is the correct answer.

18. Acetic acid is a weak acid and sodium acetate is the salt of its conjugate base. Hence, acetic acid and sodium acetate forms a buffer solution whose pH can be calculated by using Henderson hesselbatch equation:

pH = pKa + log([conjugate base]/[weak acid])

=> pH = pKa + log([sodium acetate]/[acetic acid])

=> pH = 4.75 + log(0.20M / 0.35M)

=> pH = 4.5069 or 4.51 (approx)

Hence, Option-C i.e, 4.51 is the correct answer.

19. The reaction is spontaneous if the value gibbs free energy (∆G) is negative. The formula for gibbs free energy is:

∆G = ∆H - T∆S

When ΔH is negative and ΔS is positive, the sign of ΔG will always be negative, and the reaction will be spontaneous at all temperatures

Hence, Option-B i.e, all temperatures is the correct answer.

20. During titration, at complete neutralization,

No of moles of acid = No of moles of base

As, Molarity = no of moles/Volume

=> No of moles = Molarity * Volume

Thus, ( M * V) HCl = ( M * V) NaOH

M(HCl) * 25.87 mL = 0.1146M * 28.16 mL

=> M(HCl) = 0.1247 M

Hence, HCl concentration is 0.1247 M. Option-c is the correct answer.