A chemist mixes of methylamine at with of hydrochloric acid

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A chemist mixes 25.0 mL25.0\,\text{mL} of 0.300 M0.300\,\text{M} methylamine (CH3NH2,  Kb=4.4×10−4(CH_3NH_2,\; K_b = 4.4\times10^{-4} at 25∘C)25^\circ\text{C}) with 10.0 mL10.0\,\text{mL} of 0.200 M0.200\,\text{M} hydrochloric acid. The temperature is maintained at 25∘C25^\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. mL100.\,\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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