# Acid Base Reactions Help Plz (1 Viewer)

#### csi

##### Member
1. Which of the following is not basic:

A. SO4 2-
B. NO3 -
C. HPO4 2-
D. NH3

2. Kb=4.3*10^-4, for the dissociation of ethylamine (C2H5NH2). Calculate Ka for its conjugate acid.

3. The solubility of CO2 is pure water at 25 degrees celsius and 0.1 atm pressure is 0.0037M. Assuming that all of the dissolved CO2 is in the form of carbonic acid H2CO3, which is produced by: CO2 (aq) +H2O <->H2CO3 (aq). If the acid dissociation constants, Ka for the reaction is 4.3*10^-7, what is the pH of a 0.0037M solution of H2CO3?

THANKS!!!

#### CM_Tutor

##### Moderator
Moderator
1. A compound acting as a base must be able to accept a proton, and the resulting acid form needs to plausibly exist in water (or the relevant solvent). So, for example, though I can write:

Cl- (aq) + H2O (l) <-----> HCl (aq) + OH- (aq)

and show the chloride ion acting as a base, the products are a strong acid (HCl) and hydroxide ions, which would immediate react back to the starting materials. Practically, the reaction occurs only from right to left:

Cl- (aq) + H2O (l) <----- HCl (aq) + OH- (aq)

and the chloride ion is not basic.

For the same reason, the nitrate ion is also not basic. All of the other species will be weak bases and form weak acids.

2. Kb(base form) x Ka(acid form) = Kw.

3. Use an ICE table for the ionisation of carbonic acid to the hydrogencarbonate ion

\bg_white \begin{align*} K_a &= \frac{x^2}{0.0037 - x} \\ \implies \quad x &= \sqrt{0.0037 \times 4.3 \times 10^{-7}} = 3.9887... \times 10^{-5} \\ \implies \quad \text{pH} &= 4.40 \end{align*}

csi

#### csi

##### Member
1. A compound acting as a base must be able to accept a proton, and the resulting acid form needs to plausibly exist in water (or the relevant solvent). So, for example, though I can write:

Cl- (aq) + H2O (l) <-----> HCl (aq) + OH- (aq)

and show the chloride ion acting as a base, the products are a strong acid (HCl) and hydroxide ions, which would immediate react back to the starting materials. Practically, the reaction occurs only from right to left:

Cl- (aq) + H2O (l) <----- HCl (aq) + OH- (aq)

and the chloride ion is not basic.

For the same reason, the nitrate ion is also not basic. All of the other species will be weak bases and form weak acids.

2. Kb(base form) x Ka(acid form) = Kw.

3. Use an ICE table for the ionisation of carbonic acid to the hydrogencarbonate ion

\bg_white \begin{align*} K_a &= \frac{x^2}{0.0037 - x} \\ \implies \quad x &= \sqrt{0.0037 \times 4.3 \times 10^{-7}} = 3.9887... \times 10^{-5} \\ \implies \quad \text{pH} &= 4.40 \end{align*}
Thanks

#### Eagle Mum

##### Active Member
1. Which of the following is not basic:

A. SO4 2-
B. NO3 -
C. HPO4 2-
D. NH3
Technically, Q1 should have asked which of the species is not an Arrhenius base or ‘not basic in an aqueous solution’. Otherwise, the question should have asked which of the species is the ‘least basic’. The table (https://www.slideshare.net/mobile/hilkianho/acid-base-table) comparing the strengths of different acids is helpful to work out what would happen in water or other solvents. Of the species listed in the question, only HNO3/NO3- has a higher Ka than H3O+/H2O. The other species in the question are lower in the inserted table. As CM_Tutor points out, HCl is an even stronger acid.

NO3- can accept a proton from a stronger acid such as HBr, in a gas phase (where H2O is not present and so can’t react with either the substrate or product acid) so by definition, NO3- can be a base (Davidson, J.A., Fehsenfeld, F.C. and Howard, C.J. (1977), The heats of formation of NO3− and NO3− association complexes with HNO3 and HBr . Int. J. Chem. Kinet., 9: 17-29. https://doi.org/10.1002/kin.550090104).

That’s why different definitions [Arrhenius, Brønsted-Lowry & Lewis) of acid & base were introduced.

 ​ Acid​ ​ Base​ Ka​ Name​ Formula​ Formula​ Name​ Large​ Perchloric acid​ HClO4​ ClO4 -​ Perchlorate ion​ 3.2 * 109​ Hydroiodic acid​ HI​ I-​ Iodide​ 1.0 * 109​ Hydrobromic acid​ HBr​ Br-​ Bromide​ 1.3 * 106​ Hydrochloric acid​ HCl​ Cl-​ Chloride​ 1.0 * 103​ Sulfuric acid​ H2SO4​ HSO4 -​ Hydrogen sulfate ion​ 2.4 * 101​ Nitric acid​ HNO3​ NO3 -​ Nitrate ion​ --------​ Hydronium ion​ H3O+​ H2O​ Water​ 5.4 * 10-2​ Oxalic acid​ HO2C2O2H​ HO2C2O2-​ Hydrogen oxalate ion​ 1.3 * 10-2​ Sulfurous acid​ H2SO3​ HSO3 -​ Hydrogen sulfite ion​ 1.0 * 10-2​ Hydrogen sulfate ion​ HSO4 -​ SO4 2-​ Sulfate ion​ 7.1 * 10-3​ Phosphoric acid​ H3PO4​ H2PO4 -​ Dihydrogenphosphate ion​ 7.2 * 10-4​ Nitrous acid​ HNO2​ NO3 -​ Nitrite ion​ 6.6 * 10-4​ Hydrofluoric acid​ HF​ F -​ Fluoride ion​ 1.8 * 10-4​ Methanoic acid​ HCO2H​ HCO2 -​ Methanoate ion​ 6.3 * 10-5​ Benzoic acid​ C6H5COOH​ C6H5COO-​ Benzoate ion​ 5.4 * 10-5​ Hydrogen oxalate ion​ HO2C2O2-​ O2C2O2 2-​ Oxalate ion​ 1.8 * 10-5​ Ethanoic acid​ CH3COOH​ CH3COO​ Ethanoate (acetate) ion​ 4.4 * 10-7​ Carbonic acid​ CO3 2-​ HCO3 -​ Hydrogen carbonate ion​ 1.1 * 10-7​ Hydrosulfuric acid​ H2S​ HS-​ Hydrogen sulfide ion​ 6.3 * 10-8​ Dihydrogen phosphate ion​ H2PO4 -​ HPO4 2-​ Hydrogen phosphate ion​ 6.2 * 10-8​ Hydrogen sulfite ion​ HS-​ S2-​ Sulfite ion​ 2.9 * 10-8​ Hypochlorous acid​ HClO​ ClO-​ Hypochlorite ion​ 6.2 * 10-10​ Hydrocyanic acid​ HCN​ CN-​ Cyanide ion​ 5.8 * 10-10​ Ammonium ion​ NH4 +​ NH3​ Ammonia​ 5.8 * 10-10​ Boric acid​ H3BO3​ H2BO3 -​ Dihydrogen carbonate ion​ 4.7 * 10-11​ Hydrogen carbonate ion​ HCO3 -​ CO3 2-​ Carbonate ion​ 4.2 * 10-13​ Hydrogen phosphate ion​ HPO4 2-​ PO4 3-​ Phosphate ion​ 1.8 * 10-13​ Dihydrogen borate ion​ H2BO3-​ HBO3 2-​ Hydrogen borate ion​ 1.3 * 10-13​ Hydrogen sulfide ion​ HS-​ S 2-​ Sulfide ion​ 1.6 * 10-14​ Hydrogen borate ion​ HBO3 2-​ BO3 3-​ Borate ion​ ---------​ water​ H2O​ OH-​ Hydroxide​

Last edited:
csi