Chemistry Labs

International Chemistry Olympiad · 1994

Problems

  1. Problem 1Lactic acid (HL, monoprotic, Ka(HL)=1.4×10−4K_{a}(HL) = 1.4\times10^{-4}) is formed in muscles during intense activity; in blood it is neutralised by hydrogen carbonate. Carbonic acid has Ka1=4.5×10−7K_{a1} = 4.5\times10^{-7} and Ka2=4.7×10−11K_{a2} = 4.7\times10^{-11}; all COX2\ce{CO2} stays dissolved. (1.1) Calculate the pH of a 3.00×10−33.00\times10^{-3} M solution of HL. (1.2) Calculate the equilibrium constant of HL+HCOX3X−→HX2COX3+LX−\ce{HL + HCO3^- -> H2CO3 + L^-}. (1.3) 3.00×10−33.00\times10^{-3} mol of HL is added to 1.00 dm31.00\ \mathrm{dm^3} of 0.024 M NaHCOX3\ce{NaHCO3} (no volume change, HL fully neutralised): (i) pH of the NaHCOX3\ce{NaHCO3} solution before addition? (ii) pH after addition? (1.4) Blood at pH 7.40 with [HCOX3X−]=0.022[\ce{HCO3^-}] = 0.022 M falls to pH 7.00 after exercise: how many moles of HL were added per litre? (1.5) In saturated CaCOX3\ce{CaCO3} solution pH = 9.95; calculate the solubility and show Ksp≈5×10−9K_{sp} \approx 5\times10^{-9}. (1.6) Find the maximum free [CaX2+][\ce{Ca^{2+}}] in the pH 7.40 blood of part 1.4.Solutions: 1