Consider the following data on some weak acids and weak bases: acid base name fo

Question

Consider the following data on some weak acids and weak bases: acid base name formula name formula hydrocyanic acid HCN4.9x 1010 hydrofluoric acid HF6.8x 10 methylamine CH3NH2 4.4x 104 ammoniaNH | 1.8 x 10-5 3 Use this data to rank the following solutions in order of increasing pH. In other words, select a '1' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so orn solution pH 0.1 M NH4CI choose one 0.1 M KCN choose one ? 0.1 M NalF choose one 0.1 M CH3NH3Br choose one

Explanation / Answer

Ka of HCN = 4.9 * 10-10

Ka of HF = 6.8 * 10-4

Ka of CH3NH2 = Kw / Kb = 10-14 / (4.4 * 10-4) = 2.27 * 10-11

Ka of NH3 = 10-14 / (1.8*10-5) = 5.56 * 10-10

Higher is Ka higher is the production of H+ ions and hence lower is the pH.

(OR) Lower is the Ka lower is the concentration of H+ and hence higher is the pH.

So,

The increasing order of pH of the given solutions is,

HF < HCN < NH3 < CH3NH2

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