Finding the pH of a buffer solution, Lecture notes of Chemistry

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Finding the pH of a buffer solution Finding the OH of a buffer can be a rather complicated task. The best way to learn how to solve questions like these is to take a look at an example. We have 20cm? of 2 moldm~* CH;COOH. We add 5cm* of 0.5 moldm~* NaOH to it. What is the final DH? Step 1: Find the remaining no. of moles of CH; COOH CH;COOH + NaOH > CH;COO Na®* + H,0 Original amt of CH;COOH = 730, (2) = 0.04 mol ae _ 5 _ Original amt of NaOH = z6q (0.5) = 0.0025 mol Remaining CH;COOH = 0.04 - 0.0025 = 0.0375 mol Step 2: Find the new concentration of CH; COOH _ 0.0375 _ 3 New conc. = Gos 7 1.5 moldm Step 3: Use the Ka constant to find [H*] and then find the pH. Ka of CH;COOH =1.8 x 10° CH,COO |[H*] _ s_ [ Ka =1.8xX107° = CH,COOH, As one NaOH reacts with one CH;COOH, one CH;COO7 is formed for every NaOH that reacts. Thus, 0.0025 mol of NaOH produces 0.0025mol of CH;COO. 7 — 0.0025 _ 3 [CH,COO |= 035 = O01 moldm [H+] = [CH;COOH| __ 1.5(1.8x10°) _ [CH,COO-| 0.1 BH} =lo0,,2/*%10 —=S.57 24 xKIO-