The hydrogen ion concentration of a 10^-8M HCl aqueous solution at 298K(Kw=10^-14) is? How's the answer 1.0525×10^-7 M I'm getting 1.1×10^-7?

HCl completely ionizes, and thus [H⁺] = 1 × 10⁻⁸ M
The concentration is very low, and thus the self-ionization of water must be considered.

H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)
Assume that water self-ionizes to give y M of H⁺ ions and y M of OH⁻ ions.

Kw = [H⁺][OH⁻]
[y + (1 × 10⁻⁸)] y = 1 × 10⁻¹⁴
y² + (1 × 10⁻⁸)y - 1 × 10⁻¹⁴ = 0
y = 9.512 × 10⁻⁸ or y = -1.051 × 10⁻⁷ (rejected)

[H⁺] = (1 × 10⁻⁸) + (9.512 × 10⁻⁸) = 1.0512 × 10⁻⁷ M