A solution of sodium carbonate was prepared by dissolving a known mass of anhydrous sodium carbonate (Na2CO3) in deionised water in a beaker and transferring it to a F 500 cm³ flask like shown in the diagram - Leaving Cert Chemistry - Question 2 - 2015

One precaution that can be taken is to rinse the beaker with deionised water and add the washings to the volumetric flask (F). This ensures that any sodium carbonate residue sticking to the sides of the beaker is transferred into the flask.

In the sketch of flask F, label the bottom of the curved meniscus touching line L, which indicates the exact fill level of 500 cm³. The meniscus should be curved, with a clear point at which it meets the line.

Flask F must be inverted several times to ensure thorough mixing of the sodium carbonate solution. This creates a homogeneous solution, preventing concentration gradients where parts of the solution might be more concentrated than others.

To find the mass of Na2CO3 needed, use the molarity formula:

1. Calculate the number of moles required: $ext{Moles} = ext{Concentration} imes ext{Volume}$ $ext{Moles} = 0.05 imes 0.5 = 0.025 ext{ moles}$

2. Calculate the molar mass of Na2CO3:

   • Na: 23 g/mol (2 Na = 46 g)
   • C: 12 g/mol
   • O: 16 g/mol (3 O = 48 g) $ext{Molar mass of Na2CO3} = 46 + 12 + 48 = 106 ext{ g/mol}$

3. Find the mass of Na2CO3: $ext{Mass} = ext{Moles} imes ext{Molar mass}$ $ext{Mass} = 0.025 imes 106 = 2.65 ext{ g}$

The piece of equipment used to measure out the sodium carbonate solution is a pipette. It provides accurate and precise volume measurements necessary for titrations.

A suitable indicator for this titration is methyl orange or phenolphthalein. These indicators change color at the endpoint of the titration.

For phenolphthalein, the color change is from yellow (orange/pink) in acidic solution to pink (or colourless) in neutral solution. This indicates the endpoint of the titration.

To find the concentration of the hydrochloric acid:

1. Use the titration equation and the volumes: $ext{Moles of Na2CO3} = ext{Volume} imes ext{Concentration} = 0.025 imes 0.05 = 0.00125 ext{ moles}$

2. According to the reaction: $2HCl + Na2CO3 → 2NaCl + H2O + CO2$ The mole ratio of HCl to Na2CO3 is 2:1, so:

   • Moles of HCl = 2 * moles of Na2CO3 = 2 * 0.00125 = 0.0025 moles.

3. Calculate the concentration (C) using: C = rac{ ext{Moles}}{ ext{Volume in litres}} Here, volume of HCl = rac{22.7 ext{ cm}^3}{1000} = 0.0227 ext{ L}; C = rac{0.0025}{0.0227} ext{ mol/L} = 0.1105 ext{ mol/L}. Therefore, the concentration of the hydrochloric acid solution is approximately 0.11 mol/L.