1673 What is the pH of a solution that is 0.10 M CH,NH2 (methylamine) and 0.15 M

Question

1673 What is the pH of a solution that is 0.10 M CH,NH2 (methylamine) and 0.15 M CH;NH,CI (methylammonium chloride)? 16.74 What is the pH of a solution that is 0.15 MC2HNH2 (ethylamine) and 0.10 M CHNH,Br (ethylammonium bromide)? Buffers 16.75/ A buffer is prepared by adding 39.8 mL of 0.75 M NaF to 38.9 mL of 0.28 M HF. What is the pH of the fina solution? 16.76 A buffer is prepared by adding 115 mL of 0.30N NH, to 135 mL of 0.15 M NH,NO3. What is the pH of tl final solution? 1 6.77 What is the pH of a buffer solution that is 0.10 NH, and 0.10 MNH,+? What is the pH if 12 mL of 0.20 hydrochloric acid is added to 125 mL of buffer?

Explanation / Answer

Methylamine is the base and the methylammonium chloride is the acid.

We can write the equation
CH3NH2 + H2O <---> CH3NH3+ + OH-

Equilibrium constant of methylamine, Kb = [CH3NH3+][OH-]/[CH3NH2] (Kb for methylamine = 4.4 X 10-4)

4.4 X 10-4 = 0.15 M [OH-]/ 0.10M

[OH-] = [4.4 x 10-4 X 0.10]/0.15 =  2.933 X 10-4

pOH = -log[OH-] = - log( 2.933 X 10-4) = 3.53

We know, pH + pOH = 14

Therefore, pH = 14 - 3.53 = 10.47

pH of the solution is 10.47

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