What is an isoelectronic series? Consider the series: Na+, Ne, Mg2+, F− and arrange the species in increasing order of size. Explain the order.

During the electrolysis of water, oxygen gas is produced at the anode:

$$2\text{H}_{2}\text{O}_{(l)} \longrightarrow \text{O}_{2(g)} + 4\text{H}^{+}_{(aq)} + 4\text{e}^{-}$$

From this equation, 1 mole of O$_{2}$ requires 4 moles of electrons.

Given that 1 Faraday (F) = 1 mole of electrons = 96500 C.

$$\text{Quantity of electricity} = \text{Moles of electrons} \times \text{Faraday constant}$$ $$\text{Quantity of electricity} = 0.48 \text{ mol e}^{-} \times 96500 \text{ C/mol e}^{-} = 46320 \text{ C}$$

The quantity of electricity required is $\boxed{\text{46320 C}}$.

3(e)(i)
Aqueous solutions of ammonia (a weak base) and ethanoic acid (a weak acid) are weak electrolytes because they only partially ionize in water, producing a low concentration of ions. When these two solutions are mixed, they react to form ammonium ethanoate (CH$_{3}$COONH$_{4}$), which is a salt. Salts are strong electrolytes because they dissociate completely into ions (CH$_{3}$COO$^{-}$ and NH$_{4}^{+}$) in water, leading to a high concentration of mobile ions that can conduct electricity effectively.

3(e)(ii)
Two physical properties of liquids that could be increased as a result of hydrogen bonding are:

• Boiling point: More energy is required to overcome the stronger intermolecular forces (hydrogen bonds) to vaporize the liquid.
• Viscosity: Stronger intermolecular forces make the liquid more resistant to flow.
• Surface tension: Stronger intermolecular forces at the surface lead to higher surface tension.

4(a)(i)
The stages involved in the operation of a mass spectrometer are:

1. Vaporization and Ionization: The sample is vaporized and then converted into positive ions.
2. Acceleration: The ions are accelerated by an electric field.
3. Deflection: The accelerated ions are deflected by a magnetic field based on their mass-to-charge ratio (m/z).
4. Detection: The deflected ions are detected, and their abundance is recorded.

4(a)(ii)
Two kinds of information that could be obtained from a mass spectrum of an element are:

• The number of isotopes present in the element.
• The relative abundance of each isotope.
• The relative atomic mass of the element.

4(b)(i)

Ethanoic acid (CH$_{3}$COOH) is a weak acid:

$$\text{CH}_{3}\text{COOH}_{(aq)} \rightleftharpoons \text{CH}_{3}\text{COO}^{-}_{(aq)} + \text{H}^{+}_{(aq)}$$

Initial concentration of CH$_{3}$COOH = 0.1 mol dm$^{-3}$. Let x be the concentration of H$^{+}$ ions at equilibrium.

$$K_{a} = \frac{[\text{CH}_{3}\text{COO}^{-}][\text{H}^{+}]}{[\text{CH}_{3}\text{COOH}]}$$ $$1.70 \times 10^{-5} = \frac{x \cdot x}{0.1 - x}$$

Since K$_{a}$ is small, we can approximate $0.1 - x \approx 0.1$.

$$1.70 \times 10^{-5} = \frac{x^2}{0.1}$$

$$x^2 = 1.70 \times 10^{-5} \times 0.1 = 1.70 \times 10^{-6}$$ $$x = \sqrt{1.70 \times 10^{-6}} = 1.3038 \times 10^{-3} \text{ mol dm}^{-3}$$

So, $[\text{H}^{+}] = 1.3038 \times 10^{-3} \text{ mol dm}^{-3}$.

$$pH = -\log_{10}[\text{H}^{+}] = -\log_{10}(1.3038 \times 10^{-3})$$ $$pH \approx 2.88$$

The pH of the ethanoic acid at the start of the reaction is $\boxed{\text{2.88}}$.

4(b)(ii)
The balanced equation for the reaction between ethanoic acid and sodium hydroxide is:

$$\text{CH}_{3}\text{COOH}_{(aq)} + \text{NaOH}_{(aq)} \longrightarrow \text{CH}_{3}\text{COONa}_{(aq)} + \text{H}_{2}\text{O}_{(l)}$$

4(b)(iii)
A solution of sodium ethanoate (CH$_{3}$COONa) is alkaline because it is the salt of a weak acid (ethanoic acid) and a strong base (sodium hydroxide). When dissolved in water, the salt dissociates completely into Na$^{+}$ and CH$_{3}$COO$^{-}$ ions. The ethanoate ion (CH$_{3}$COO$^{-}$) is the conjugate base of a weak acid, and it hydrolyzes in water to produce hydroxide ions (OH$^{-}$), making the solution alkaline:

$$\text{CH}_{3}\text{COO}^{-}_{(aq)} + \text{H}_{2}\text{O}_{(l)} \rightleftharpoons \text{CH}_{3}\text{COOH}_{(aq)} + \text{OH}^{-}_{(aq)}$$

The Na$^{+}$ ion, being the conjugate acid of a strong base, does not hydrolyze significantly.