Download L11 worksheet Ans

CHEM1611 Worksheet Answers – Lecture 11 1. Complete the table by writing the conjugate pair. Write the formula of the conjugate bases Write the formula of the conjugate acids H3O+ H2O OH– H2O H2SO4 HSO4– H2O H3O+ HClO4 ClO4– Cl– HCl CH3COOH CH3COO– NH3 NH4+ 2. Calculate the pH of the following three solutions: i)
0.001 M HNO3 [H+] = 0.001 M ∴pH ii)
0.001 M NaOH [OH–] = 0.001 M ∴pOH = 3.0 = 3.0 and pH = 14 – pOH = 11.0 iii) The solution resulting from mixing 400 mL of 0.05 M HCl with 600 mL of 0.05 M NaOH. Excess of 200 mL of 0.05 M NaOH so the amount of OH– in excess is 0.05 M x 0.2 L = 0.01 mol The total volume is 1000 mL so [OH–] = 0.01 mol / 1 L = 0.01 M pOH = 2.0 and pH = 12.0 iv) What is the [H+] of a solution with a pH of 4.5 ? pH = -­‐log[H+] ∴ [H+] = 10–pH = 10–4.5 = 3.2 x 105 M = 3 x 105 M to one significant figure. (Note in a log term such as pH, the number before the decimal point refers to the exponential value of the original number and so is not counted as a significant figure. The mantissa of a log term – the numbers after the decimal point – refers to the value of the original number and should be quoted to the appropriate number of significant figures.) v) What is the [OH–] of a solution with a pH of 12.2 ? pOH = 14.0 – 12.2 = 1.8 ∴ [OH–] = 10–pOH = 10–1.8 = 1.6 x 10–2 M = 0.02 M to one significant figure. The strength of acids and bases. • The smaller the value of pKa the stronger the weak acid. • The smaller the value of pKb the stronger the weak base. 1. Order the following compounds in increasing acid strength HCl, H2CO3 (pKa = 6.35), HClO2 (pKa = 1.95), HF (pKa = 3.17), HCN (pKa = 9.21) HCN < H2CO3 < HF < HClO2 < HCl 2. Order the following compounds in increasing base strength NaOH, HPO42— (pKb 1.6), F— (pKb 10.8), CH3COO— (pKb 9.2), NH3 (pKb 4.8) F— < CH3COO— < NH3 < HPO42— < OH— The stronger an acid, the weaker its conjugate base. 3. Write the conjugate bases of the acids in Q1 in order of increasing base strength. Cl— < ClO2— < F— < HCO32— < CN— 4. Write the conjugate acids of the bases in Q2 in order of increasing acid strength. H2O < H2PO4— < NH4+ < CH3COOH < HF Taken from 2015 exam 5. Which one of the following is a conjugate acid-­‐base pair? A
H3O+ and OH– B
HSO4– and SO42– C
HSO3– and SO3 D
H2SO4 and SO42– E
NH4+ and SO42– Adapted from 2015 exam. 6. Consider the following compounds and their approximate pKa values Compounds pKa CH3COOH ~ 5 ~ 10 OH
CH3CH2OH ~ 17 Rank the compounds in order of increasing acidity and explain the reason for the trend. CH3CH2OH < C6H5OH < CH3COOH In all cases an O-­‐H bond is broken, so that does not determine the degree of acidity. The trend is associated with the stability of the conjugate base. CH3COO— is a resonance stabilised ion and so much more stable than CH3CH2O— in which the charge is localised on the oxygen. In C6H5O— there is some delocilisation of the charge into the π-­‐system of the adjacent benzene ring giving an intermediate stability.