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USTH- Analytical chemistry- Fall 2013 Homework assignments – Lecture Lecture 4 nd (for 2 BP and WEO students)
(Find more exercises in the book: D. Harvey, Modern analytical chemistry, McGraw Hill Higher Education, 2000) 1. Acid- base equilibrium Problem 1: Calculate the pH of a- 1.0x10-8 HBr b- A solution prepared by dissolving 1.23 g of 2- nitrophenol ( FM 139.11) in 0.250 L. c- A 0.050 M sodium butanoate. Problem 2: A solution contains 63 different conjugate acid- base pair. Among them is acrylic acid and acrylate ion, with the equilibrium ratio [acrylate]/[acrylic acid]= 0.75. What is pH of the solution. + Problem 3: Find the pH and concentrations of (CH 3)3,N and (CH 3)3 NH in a 0.060 M solution of trimethylamine. Problem 4: Which of the following acids would be most suitable for preparing a buffer of pH 3.10: (i) hydrogen peroxide; (ii) propanoic acid; (iii) cyanoacetic a cid; (iv) 4aminobenzenesulfonic acid? Problem 5: A buffer was prepared by dissolving 0.100 mol of the weak acid HA (K a = 1.00 X 105 + ) plus 0.050 mol of its conjugate base Na A in 1.00 L. Find the pH. -4 -8 Problem 6: Consider the diprotic acid H 2A with K 1 = 1.00 X 10 and K 2 = 1.00 X l0 . Find the 2 pH and concentrations of H 2A, HA , and A in (a) 0.100 M H 2A; (b) 0.100 M NaHA; (c) 0.100 M Na 2A. Problem 7: How many milliliters of 0.202 M NaOH should be added to 25.0 mL of 0.0233 M salicylic acid (2-hydroxybenzoic acid) to adjust the pH to 3.50? Problem 8: a) Which two of the following compounds would you mix to make a buffer of pH 7.45: H 3PO4, (FM 98.00), NaH 2PO4 (FM 119.98), Na 2HPO4 (FM 141.96), and Na 3PO4 (FM 163.94)? (b) If you wanted to prepare 1.00 L of buffer with a total phosphate concentration of 0.0500 M, how many grams of each of the two selected compounds would you mix together? (c) If you did what you calculated in i n part (b), you would not get a pH of exactly 7.45. Explain how you would really prepare this buffer in the lab. 2. Complexon equilibrium 2+ Problem 9: Consider the reactions of Fe with the amino acid glycine: 2+
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Fe + G FeG 2+
+
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Fe + 2G FeG2(aq) Fe2+ + 3G - FeG3-; 2+ + + Fe + H20 : FeOH + H
+H3 NCH2CO2H glycine, H 2G+ pK 1 = 2.350, pK 2 = 9.778 Suppose that 0.050 mol of FeG 2 is dissolved in 1.00 L and enough HCl is added to adjust the pH to 8.50. Use activity coefficients to find the composition of the solution. ( get an ionic strength of 0.01M)What fraction of iron is in each of its forms and what fraction of glycine is in each of its forms? From the distribution of species, explain the principal chemistry that requires addition of HCl to obtain a pH of 8.50. 2. Pricipitation equilibrium 2+ Problem 10: a) Find the concentration of Cu in equilibrium with CuBr(s) and 0.10MBr 2+ -5 b) Find [Cu ] in a solution saturated with Cu 4(OH)6(SO4) if [OH ] is fixed at 1.0 X l0 M. Note that Cu4(OH)6(SO4) gives 1 mol of SO 42- for 4 mol of Cu 2+. 2+ 2-69 Cu4(OH)6(SO4)(s) 4Cu + 60H + S04 K sp = 2.3 X 10 Problem 11: The planet Aragonose (which is made mostly of the mineral aragonite, whose composition is CaCO 3) has an atmosphere containing methane and carbon dioxide, each at a pressure of 0.10 bar. The oceans are saturated with aragonite and have a concentration of H + equal to 1.8 X l0-7 M. Given the following equilibria, calculate how many grams of calcium are contained in 2.00 L of Aragonose seawater. CaCO3(s,aragonite) Ca2+(aq) + CO32-(aq) K sp = 6.0 X l0 -9 -2 C02(g) 3 CO2 (aq) K C02= 3.4 x 10 + -7 CO2(aq) + H2O(l) HCO3 (aq) + H (aq) K 1 = 4.5 x 10 HCO3-(aq) H+ (aq) + CO 32- (aq) K 2= 4.7x10-11