Determine the K_b for CN^- at 25degree C. The K_a for HCN is 4.9 times 10^-10. A

Question

Determine the K_b for CN^- at 25degree C. The K_a for HCN is 4.9 times 10^-10. A) 4.9 times 10^-14 B) 2.3 times 10^-9 C) 1.4 times 10^-5 D) 2.0 times 10^-5 E) 3.7 times 10^-7 Which of the following is a Lewis base? A) BF_3 B) NH_3 C) HI L D) Na^+ Identify a good buffer. A) Small amounts of both a weak acid and its conjugate base B) Significant amounts of both a strong acid and a strong base C) Small amounts of both a strong acid and a strong base D) Significant amounts of both a weak acid and its conjugate base Which of the following is TRUE? A) An effective buffer has a (base)/(acid) ratio in the range of 10 - 100. B) A buffer most resistant to pH change when (acid) = [conjugate base] C) An effective buffer has very small absolute concentrations of acid and conjugate base. D) A buffer can not be destroyed by adding too much strong base. It can only be destroyed by adding too much strong acid. E) None of the above arc true. Which of the following is TRUE? A) The equivalence point is where the amount of acid equals the amount of base during any acid-base titration. B) At the equivalence point, the pH is always 7. C) An indicator is not pH sensitive. D) A titration curve is a plot of pH vs, the (base]/[acid] ratio. E) None of the above arc true. A 100.0 mL sample of 0.18 M HNO_3 is titrated with 0.27 M LiOH. Determine the pH of the solution after the addition of 66.67 mL of LiOH (this is the equivalence point). A) 0.97 B) 13.03 C) 2.76 D) 11.24 E) 7.00

Explanation / Answer

6] Ka * Kb = 10^-14

given Ka for HCN = 4.9 * 10^-10

Then Kb =2.04 * 10^-5

Answer is D

7] A Lewis base is an electron-pair donor. Because the Lewis base donates electrons, we can also say that it is a nucleophile, which is a substance that behaves as an electron donor .

Example : H2O and NH3

Answer is B

8] D

9] B

10] A

11] HNO3 ----> 0.18 M and 100 ml

LiOH -----> 0.27 M and 66.7 ml

Moles of HNO3 = MV / 1000 = 0.018

Moles of LiOH = 0.018

Moles of HNO3 = Moles of LiOH

so it forms salt and water by consuming all moles of acid and base .

Therefore pH =7

Answer is E.

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