1. All the following act as a Lewis acid in water except

a. SO3
b. BH3
c. Sn4+
d. Na2O *** I think c or d.

2. Which acts as a Lewis acid but neither Arrhenius nor Brønsted-Lowry?
a. Fe3+(aq)
b. NH3(aq)
c.HCOOH(aq)
d. HClO(aq)

3. An unknown solution forms a precipitate when the pH is raised but does not when sulfate is added. The cation present could be which of the following?
a. Li+
b. Ba2+
c. K+
d. Mg2+

d

a
d

To answer these questions, we need to understand the concepts of Lewis acids and bases, Arrhenius acids and bases, Brønsted-Lowry acids and bases, as well as solubility rules.

1. To determine which of the given options acts as a Lewis acid in water, we need to identify the substance that can accept a pair of electrons. In Lewis acid-base theory, a Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. When a substance acts as a Lewis acid in water, it accepts an electron pair from water molecules.

a. SO3: Sulfur trioxide can react with water to form sulfuric acid (H2SO4), where it accepts an electron pair. Hence, it acts as a Lewis acid.
b. BH3: Borane (BH3) can also react with water to form boronic acid (B(OH)3), where it accepts an electron pair. Therefore, it acts as a Lewis acid.
c. Sn4+: Tin(IV) ion has four empty orbitals and can accept electron pairs, making it a Lewis acid.
d. Na2O: Sodium oxide (Na2O) is an ionic compound that dissociates in water to form Na+ and O2- ions. Since it does not have any empty orbitals to accept electron pairs, it does not act as a Lewis acid.

Thus, the correct answer is option d. Na2O.

2. To determine which substance acts as a Lewis acid but neither Arrhenius nor Brønsted-Lowry acid, let's examine the definitions of Arrhenius and Brønsted-Lowry theories.

Arrhenius acid: An Arrhenius acid is a substance that dissociates in water to produce hydrogen ions (H+).

Brønsted-Lowry acid: A Brønsted-Lowry acid is a substance that donates a proton (H+) to a base.

a. Fe3+(aq): This is not an Arrhenius acid because it does not dissociate to produce H+ ions. However, it can act as a Lewis acid since it can accept electron pairs.
b. NH3(aq): Ammonia is a base according to the Arrhenius definition because it accepts H+ ions from water. It is also a Brønsted-Lowry base, as it can donate an electron pair to an acid.
c. HCOOH(aq): Formic acid can act as an Arrhenius acid because it dissociates in water to produce H+ ions. It is also a Brønsted-Lowry acid, donating a proton to a base.
d. HClO(aq): Hypochlorous acid can act as an Arrhenius acid since it dissociates to produce H+ ions. It is also a Brønsted-Lowry acid, donating a proton to a base.

Therefore, none of the given options in question 2 acts as a Lewis acid but neither Arrhenius nor Brønsted-Lowry acid.

3. The question asks us to determine the cation present in an unknown solution that forms a precipitate when the pH is raised but does not when sulfate is added. Let's consider the given cations and analyze their behavior.

a. Li+: Lithium ion (Li+) does not form an insoluble precipitate when the pH is raised. It usually forms soluble salts in water.
b. Ba2+: Barium ion (Ba2+) does form an insoluble precipitate when the pH is raised, indicating the formation of barium hydroxide (Ba(OH)2).
c. K+: Potassium ion (K+) does not form an insoluble precipitate when the pH is raised. It usually forms soluble salts in water.
d. Mg2+: Magnesium ion (Mg2+) does form an insoluble precipitate when the pH is raised, indicating the formation of magnesium hydroxide (Mg(OH)2).

Based on the given information, the cation that could be present in the unknown solution is option d. Mg2+.