Lots of questions, please help me... 70. What is the pH of a 0.23999999999999999
ID: 1057591 • Letter: L
Question
Lots of questions, please help me... 70. What is the pH of a 0.23999999999999999 M solution of methylamine (CH3NH2, Kb 4.4 x 104 at 25 oC? a. 5.6316228304985136 b. 0.61978875828839397 c. 12.002534979813147 d. 13.38021 1241711606 e. 1.9974650201868531 71. Which of following would have the highest value of absolute entropy per mole? a. water at 50 C b. water at 10°C c. ice at -1 d. 1 M NaCl at 50°C e. 1 M NaCl at 10°C 72. The standard enthalpy of vaporization of Freon-10, COCl4, is 29.82 kJ/mol at its nomal boiling point of 77oC. What is the change of entropy for 1 mol of liquid Freon-10 when it vaporizes at its normal boiling point? a. 8.5199999999999996 x 102 JK b, 1.043700000000000 x 104 JK c. 85.2 J/K d. 0.387 JK e. 3.8727272727272726 x 102 JK. 73. Solutions are made by combining equal amounts of the following. Which is a buffer? a 0.1 MHCl 0.1 M Cl b. 0.1 M HF 0.1 M Na c. 0.1 M HF 0.1 M NaOH d. 0.1 MNaF 0.05 M HF e. 0.1 M NaF 0.05 M NaExplanation / Answer
Let be the dissociation of the weak base
BOH <---> B + + OH-
initial conc. c 0 0
change -c +c +c
Equb. conc. c(1-) c c
Dissociation constant, Kb = (c x c) / ( c(1-)
= c 2 / (1-)
In the case of weak bases is very small so 1- is taken as 1
So Kb = c2
==> = ( Kb / c )
Given Kb = 4.4x10-4
c = concentration = 0.23999999999999999 M
Plug the values we get = 42.8x10-3
So the concentration of [OH-] = c
= 0.2399999999999999 x42.8x10-3
= 0.010276 M
pOH = - log [OH-]
= - log 0.010276
= 1.988168041
pH = 14 - pOH
= 14 - 1.988168041
= 12.01183196
Therefore option (c) is correct
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