Chemistry

Problem

A chemist mixes

25.0\,\text{mL}

0.300\,\text{M}

methylamine

(CH_3NH_2,\; K_b = 4.4\times10^{-4}

25^\circ\text{C})

with

10.0\,\text{mL}

0.200\,\text{M}

hydrochloric acid. The temperature is maintained at

25^\circ\text{C}

throughout the experiment. (a) Determine the number of moles of all chemical species present immediately after mixing but before any equilibrium is established. (b) Calculate the pH of the solution once equilibrium has been reached. Show all steps of your reasoning, including any equilibrium expressions used. (c) The resulting mixture is quantitatively transferred to a

100.\,\text{mL}

volumetric flask and diluted to the mark with distilled water. Calculate the new pH after dilution and explain why the pH changes by the amount you obtain.

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