Problem Page Question A chemistry graduate student is given of a ammonia solution. Ammonia is a weak base with . What mass of should the student disso

8.4g of NH₄Br the student need to dissolve into the solution.

Explanation:

Full question is:

A chemistry graduate student is given 300 mL of a 0.20 M ammonia (NH3) solution. Ammonia is a weak base with Kb = 1.8 x10^-5. What mass of NH4Br should the student dissolve in the NH3 solution to turn it into a buffer with pH = 9.10?

Using H-H expression for weak bases, you can find pH of a buffer thus:

pOH = pKb + log [BH⁺] / [B]

Where pKb is -log Kb =4.744, [BH⁺] concentration of of conjugate acid, NH₄Br and [B] concentration of weak base, NH₃ (It is possible to take the moles of both compounds and not its concentration).

As pH the student wants is 9.10, pOH is:

pOH = 14 - pH = 14 - 9.10 = 4.90

Replacing this information in H-H equation:

4.90 = 4.744 + log [NH₄Br] / [NH₃]

Moles of NH₃ in the solution are:

300mL = 0.300L × (0.20mol / L) = 0.060mol NH₃

Knowing this:

4.90 = 4.744 + log [NH₄Br] / [0.06mol]

0.156 = log[NH₄Br] / [0.06mol]

1.4322 = [NH₄Br] / [0.06mol]

0.08593 moles = [NH₄Br]

To create the buffer of pH = 9.10, you need 0.08593 moles of NH₄Br

As molar mass of NH₄Br is 97.94g/mol, mass of 0.08593 moles are:

0.08593 moles NH₄Br × (97.94g / mol) =

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