In a school laboratory a group of students carried out investigations on water from a nearby river - Leaving Cert Chemistry - Question 3 - 2015

The positive test for the nitrate ion is indicated by a brown colour ring R.

During the test, sulfuric acid (H₂SO₄) is added, which makes up the lower layer L.

The salt used as the reagent in the upper layer U is iron(II) sulfate (FeSO₄).

To measure the concentration of suspended solids, students would:

• Filter a known volume of river water through filter paper.
• Weigh the filter paper beforehand to know its mass.
• Wash the filter paper with distilled water to ensure all solids are collected.
• Dry the filter paper in an oven to remove moisture.
• Weigh the filter paper again to find the difference in mass, which represents the mass of suspended solids.

To measure the concentration of dissolved solids, students would:

• Use previously filtered water from the river in a dry beaker/container.
• Weigh the container before adding the water to know its mass.
• Evaporate the water to dryness, leaving the solids behind.
• Reweigh the container to find the difference in mass, which indicates the mass of dissolved solids.

To find the concentration of suspended solids:

• Using the mass of suspended solids found (0.03 g) and the volume of water filtered (500 cm³ or 0.5 L): ext{Concentration} = rac{0.03 ext{ g}}{0.5 ext{ L}} = 0.06 ext{ g/L}

For dissolved solids (0.13 g in 200 cm³ = 0.2 L):

1. In grams per litre: ext{Concentration} = rac{0.13 ext{ g}}{0.2 ext{ L}} = 0.65 ext{ g/L}

2. In mg per litre (1 g = 1000 mg): $0.65 ext{ g/L} = 650 ext{ mg/L} ext{ (or p.p.m.)}$