Question

the table for problem 5 is attached

S. Consider the weak acids in Table 13.2. Which acid-base pair would be best for a buffer at a pH of: a) 3.5 b) 7.3 c) 10.0 To make a buffer with pH 3.0 from HCHO2 and CHO2: a) what must the [HCHO2]/[CHO2] ratio be? 6. ep b) how many moles of HCHO2 must be added to a liter of 0.139 M NaCHO2 to give this pH? c) how many grams of NaCHO2 must be added to 350.0 mL of 0.1 59 M HCHO2 to give this pH? d) What volume of 0.236 M HCHO, must be added to 1.00 L of a 0.500 M solution o NaCHO2 to give this pH? (assume the volumes are additive.) 69 M KO r the reaction that takes p A bottle with 0.500 L of aqueous ammonia is labeled "13.2% by 0.9430 g/mL." How many grams of ammonium chloride should be added to prod. a buffer with a pH of 9.45? ammonia, dens mass An aqueous solution of 0.043 M weak acid, HX, has a pH of 3.92. What is the p the solution if 0.021 mol of KX is dissolved in one liter of the weak acid? 8. (5 Boo TABLE 13.2 Equilibrium Constants for Weak Acids and Their Conjugate Bases Acid K к, Base HSO 59 x 10- Sulfurous acid H2SO 17 x 10 SO 10 x 10 10 x 10-12 Hydrogen sulfate ion HSO4 71 x 10 H2PO 14 x 10-2 Phosphoric acid HaPO 5 x 10-12 Fe(HO),OH2 Fe(HO) 6.7 x 10- Hexaaquairon(II) ion 14 x 10- 69 x 10 F Hydrofluoric acid HE 6.0 x 10 NO2 17 x 10- Nitrous acid HNO2 5.3 x 10 19 x 10 Formic acid НСНО, Cно, 14 x 10 C H.O 71 x 10- Lactic acid НС-н,о, Benzoic acid HC,HO 6.6 x 10 CHsO2 15 x 10-0 C2Hy0 18 x 10- 5.6 x 10-10 НС-но, Acetic acid 12 x 10 Al(H-O),OHP Al(H,O) 8.3 x 10-10 Hexaaquaaluminum() ion 44 x 10- HCO H2CO 2.3 x 10- Carbonic acid 16 x 10- HPO 6.2x 10 Dihydrogen phosphate ion H2PO 6.0 x 10 so 17 x 10- HSO, Hydrogen sulfite ion 3.6 x 10 2.8 x 10 CIO- HCIO Hypochlorous acid 5.8 x 10-10 17 x 10- CN" HCN Hydrocyanic acid 18 x 10 NHA 5.6 x 10-10 NH Ammonium jon Zn(H,O) 3.0 x 10 Zn(H-O),OHt 3.3 x 10-10 Tetraaquazinc() jon 21x 10-4 47 x 10 CO2 Hydrogen carbonate ion HCO 45 x10- POPt 2.2 x 10- HPO Hydrogen phosphate ion [H]x [8 1 H'(aq) + B (ag) HB(aq) [HB] [HB] x |OH K# HB( aq ) +OH-(ag) B (ag) + H.O (B1

Answer 1

5) The most effective acid / base pairs are those that have a pKa closest to the required pH:

a) Nitrous acid has a pKa of 3.39, it can be used for this pH.

b) H2PO4- has a pKa of 7.21, it can be used for this pH.

c) HCO3- has a pKa of 10.32, it can be used for this pH.

6) a) Calculate the concentration relationship with the equation of henderson hasselbach cleared:

[HCHO2] / [CHO2-] = 10 ^ (pKa-pH) = 10 ^ (3.75 - 3) = 5.62

b) It must:

n HCHO2 = 5.62 * n CHO2- = 5.62 * 0.139 mol = 0.781 mol

c) The moles of acid are calculated:

n HCHO2 = M * V = 0.159 M * 0.35 L = 0.056 mol

The moles of CHO2- are calculated:

n CHO2- = 0.056 mol / 5.62 = 0.01 mol

The grams are calculated:

g NaCHO2 = n * MM = 0.01 mol * 68.01 g / mol = 0.68 g

d) The moles of acid are calculated:

n HCHO2 = 5.62 * 0.5 = 2.81 mol

The volume is calculated:

V = n / M = 2.81 / 0.236 = 11.91 L

7) The moles of ammonia are calculated:

n NH3 = 500 mL * 0.943 g / mL * (13.2 / 100) / 17.03 g / mol = 3.65 mol

The molar ratio is calculated:

n NH3 / n NH4 + = 10 ^ (9.45 - 9.25) = 1.58

The grams of NH4Cl are calculated:

g NH4Cl = 3.65 mol * 53.49 g / mol / 1.58 = 123.57 g NH4Cl

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