Silicon is the second‑most abundant element in the Earth’s crust, accounting for 25.7 % by weight. It is primarily found in silicate minerals and sand, the latter being largely composed of silicon dioxide (SiO₂), commonly known as silica. Determining the weight‑percentage of silicon in silica is useful for geologists, materials scientists, and anyone working with silica‑based materials.

Step 1 – Retrieve Atomic Masses

Locate the atomic masses of silicon (Si) and oxygen (O) in a reliable periodic table. Typical values are:
• Silicon (Si): 28.0855 u
• Oxygen (O): 15.9994 u

Step 2 – Compute the Molecular Mass of SiO₂

Silica’s formula, SiO₂, indicates one silicon atom bonded to two oxygen atoms. The molecular mass is therefore:

28.0855 (u Si) + 2 × 15.9994 (u O) = 60.0843 u

Step 3 – Calculate the Weight‑Percent of Silicon

Divide the atomic mass of silicon by the molecular mass of silica, then multiply by 100 to express it as a percentage:

(28.0855 / 60.0843) × 100 ≈ 46.69 %

Thus, silica contains approximately 46.69 % silicon by weight.

TL;DR (Quick Summary)

Use the same procedure to determine silicon content in hydrated silica, such as silicic acid (SiO₂·2H₂O). Include the mass of water (18 u per molecule) in the calculation:

(28.0855 / [28.0855 + 2 × 15.9994 + 2 × 18]) × 100 ≈ 29.2 %

For more details on atomic masses and element properties, consult a reputable periodic table resource.