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Properties of Buffers

AP Chemistry· difficulty 4/5

A buffer is made from 0.40 M NH3NH_3 and 0.30 M NH4ClNH_4Cl. What is the pH? (Kb(NH3)=1.8×10−5K_b(NH_3) = 1.8\times 10^{-5})

  • A

    pH≈9.38pH \approx 9.38

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  • B

    pH≈9.26pH \approx 9.26

  • C

    pH≈4.62pH \approx 4.62

  • D

    pH≈8.92pH \approx 8.92

Explanation

pKa(NH4+)pK_a(NH_4^+) = 14 - pKb(NH3)pK_b(NH_3) = 14 - 4.74 = 9.26. pH = pKa+log⁡([NH3]/[NH4+])pK_a + \log([NH_3]/[NH_4^+]) = 9.26 + log⁡(0.40/0.30)\log(0.40/0.30) = 9.26 + 0.125 = 9.38.

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