Calculate the concentration of HCl.

3)

From the equation, the mole ratio of $\text{NaOH}$ to $\text{HCl}$ is $1:1$.

Given: Volume of $\text{NaOH} = 26 \text{ cm}^3 = 0.026 \text{ dm}^3$
Concentration of $\text{NaOH} = 0.1 \text{ M}$

Since the mole ratio of $\text{NaOH}$ to $\text{HCl}$ is $1:1$,

Given: Volume of $\text{HCl} = 12.4 \text{ cm}^3 = 0.0124 \text{ dm}^3$

Rounding to three significant figures:

The concentration of HCl is $\boxed{\text{0.210 mol/dm}^3}$.

1)

According to the equation, $2 \text{ volumes of CO}$ react with $1 \text{ volume of O}_2$.
Given: Volume of $\text{CO} = 36 \text{ cm}^3$
Volume of $\text{O}_2 = 72 \text{ cm}^3$

For $36 \text{ cm}^3$ of $\text{CO}$, the required volume of $\text{O}_2$ is:

Since $18 \text{ cm}^3$ of $\text{O}_2$ is required and $72 \text{ cm}^3$ of $\text{O}_2$ is available, $\text{O}_2$ is in excess. $\text{CO}$ is the limiting reactant.

From the equation, $2 \text{ volumes of CO}$ produce $2 \text{ volumes of CO}_2$.

The gaseous mixture consists of the product ($\text{CO}_2$) and the unreacted excess reactant ($\text{O}_2$).

The total volume of the resulting gaseous mixture is $\boxed{\text{90 cm}^3}$.

4)
To prepare crystals of copper(II) chloride from copper metal:

1. Heat the copper metal in air to form copper(II) oxide. This is because copper does not react with dilute hydrochloric acid. $$\text{2Cu(s)} + \text{O}_2\text{(g)} \xrightarrow{\text{heat}} \text{2CuO(s)}$$
2. Add the black copper(II) oxide to a beaker containing dilute hydrochloric acid. Stir the mixture gently. The copper(II) oxide will react with the acid to form a blue solution of copper(II) chloride. $$\text{CuO(s)} + \text{2HCl(aq)} \to \text{CuCl}_2\text{(aq)} + \text{H}_2\text{O(l)}$$
3. Heat the solution gently to evaporate some of the water, concentrating the solution until it is saturated. This can be tested by dipping a clean glass rod into the solution and observing if crystals form on cooling.
4. Allow the concentrated solution to cool slowly at room temperature. Copper(II) chloride crystals will form.
5. Filter the crystals from the remaining solution (mother liquor) and wash them with a small amount of cold distilled water to remove impurities.
6. Dry the crystals between filter papers or in a desiccator.

5)
An isotope refers to atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons, resulting in different mass numbers. They have identical chemical properties but differ in their physical properties.