To convert the concentration of silica (SiO2) from parts per million (ppm) to moles per cubic decimeter (mol/dm³), we need to use the molar mass of SiO2 and the definition of ppm.
The molar mass of SiO2 can be calculated by summing the atomic masses of silicon (Si) and two oxygen (O) atoms:
Molar mass of SiO2 = (1 × molar mass of Si) + (2 × molar mass of O) = (1 × 28.1 g/mol) + (2 × 16 g/mol) = 60.1 g/mol
Now, let's calculate the concentration of SiO2 in mol/dm³:
Concentration (mol/dm³) = (Concentration in ppm × 10^-6) × (1 mol / molar mass of SiO2) = (25 ppm × 10^-6) × (1 mol / 60.1 g/mol) ≈ 4.16 × 10^-7 mol/dm³
Therefore, the concentration of SiO2 in mol/dm³ is approximately 4.16 × 10^-7 mol/dm³.