If a salt containing the conjugate base of a weak acid is dissolved in water, th
ID: 524671 • Letter: I
Question
If a salt containing the conjugate base of a weak acid is dissolved in water, the pH of the resulting solution is _____ (less than, equal to, greater than) seven. b) The K_sp of Ag_2CO_3 is 8.1 times 10^-12. If you want to increase the concentration of Ag^+ in a saturated solution of Ag_2CO_3, you would add (NaOH, HNO_3, NaCO_3 or AgNO_3) c) The K_a of the weak acid HA is 7.2 times 10^-6. The pH of a solution prepared by mixing 10 mLs of 0.1 M HA and 10 mLs of 0.1 M NaA is _____ (4.1, 4.8, 5.1, 6.8) The pH at the equivalence point of a titration of a weak base by a strong acid will be _____ (less than 7, equal to 7, greater than 7, equal to the pK_b of the base) e) Given the following K_sp's: BaSO_4 K_sp = 1.1 times 10^-10;; CaSO_4 Ksp = 2.4 times 10^-5; PbSO_4 K_sp = 6.3 times 10^-7. If a solution of Na_2SO_4 is added dropwise to a solution containing Ba^2+ Ca^2+ and Pb^2+, the first species to precipitate is _____ (Na_2SO_4 BaSO_4, CaSO_4, PbSO_4). If the K_a of HCN is 4.9 times 10^-10, the equilibrium constant for the reaction CN^- + H_2O HCN+ OH^- is _____. On the line below, write the reaction showing that K_2O is a basic anhydride Oxalic acid H_2C_2O_4 is a diprotoic acid. The ionization constants for oxalic acid are 6.4 times 10^-5 and 5.9 times 10^-2 The K_a for H_2C_2O_4 HC_2O_4^- H^+ is _____ The K_a for HC_2O_4^- C_2O^2-_4 + H^+ is _____ A 0.210 g sample of an acid (molar mass = 192 g/mole) is titrated with 30.5 mL of 0.108M NaOH to a phenolphthalein end point. The acid is _____ (monoprotic, diprotic, triprotic).Explanation / Answer
(a) Since the salt has conjugate base of weak acid, when it is dissolved in water it generates a basic solution. Hence its pH is greater than seven.
Conjugate base of a weak acetic acid is actate ion. When it is in water,
CH3COO- (aq.) + H2O (l) = CH3COOH (aq.) + OH- (aq.) (basic solution)
(b) THe saturated solution equation of silver carbonate is,
Ag2CO3 (s) = 2 Ag+ (aq.) + CO32- (aq.)
Additon of strong acid would increase the solubility of silver carbonate by removing carbonate ions from equilibrium.
CO32- (aq.) + HNO3 (aq.) -----------------> HCO3- (aq.) + NO3- (aq.)
(c)
pKa = - LogKa = - Log(7.2 * 10-6) = 5.14
Usin gthe formula to calculate pH of buffers,
pH = pKa + Log[salt]/[acid] = 5.1 + Log(0.1/0.1) = 5.1
(d)
less than 7. since the base is weak.
(e) BaSO4 , becuase its Ksp is small.
(f)
Kb = [HCN][OH-]/[CN-]
Multiply and divide by [H+]
Kb = ([HCN]/[H+][CN-]) * ([H+][OH-]
Kb = (1/Ka) * Kw
Kb = (1/4.9*10-10)*1.0*10-14
Kb = 2.04 * 10-5
(g)
K2O (s) + H2O (l) ----------> 2 KOH (aq.)
(h) First ionisation constant is always greater than second ionisation constant.
Ka for first reaction = 5.9 * 10-2
Ka for second reaction = 6.4 * 10-5
(i) moles of acid = 0.210 / 192 = 0.00109 mol
Moles of base = 0.108 * 30.5 / 1000 = 0.00329 mol
Ratio of mole sof base to acid = 0.00329 / 0.00109 = 3
So, the acid is triprotic aicd. Because it needs 3 moles of base for every 1 mole of acid.
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