2. A 0.10 M standard solution of sodium hydroxide (NaOH) was used to find the concentration of a given hydrochloric acid (HCl) solution by titration - Leaving Cert Chemistry - Question 2 - 2005

The hydrochloric acid (HCl) solution is normally placed in the burette (A). To rinse and fill the burette, follow these steps:

1. Rinse the burette with deionised water to remove any contaminants.
2. Rinse the burette with the HCl solution to ensure it is coated with the solution that will be used.
3. After rinsing, use a funnel to fill the burette, ensuring not to trap air bubbles.
4. Fill the burette to the desired level, making sure to read the bottom of the meniscus at eye level.

A suitable indicator for this titration is methyl orange. At the end point, the colour changes from yellow (when the solution is basic) to pink (when it becomes neutral).

Using the titration data, we can calculate the concentration of the HCl solution:

Given:

• Volume of NaOH used = 25 cm³
• Molarity of NaOH = 0.10 M
• Average titre of HCl solution = 22.6 cm³

Using the formula:

$C_1V_1 = C_2V_2$

Where:

• $C_1$ = concentration of NaOH
• $V_1$ = volume of NaOH used
• $C_2$ = concentration of HCl
• $V_2$ = average titre of HCl

Substituting the values:

$0.10 imes 25 = C_2 imes 22.6$

This leads to:

$C_2 = \frac{0.10 \times 25}{22.6} \approx 0.11 \, M$

To prepare a pure sample of sodium chloride, follow these steps:

1. Carry out the titration as described, adding the acid to the base until reaching the end point.
2. No indicator is needed at this stage, as you can monitor the colour change directly.
3. Once the reaction is complete, evaporate the solution to dryness in an evaporating dish, leaving behind pure sodium chloride.
4. Alternatively, you can heat the solution gently to speed up the evaporation process.