The reaction is CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. 2 moles of HCl react with 1 mole of CaCO₃. So, 1 mole of HCl will react with 0.5 moles of CaCO₃. Moles of CaCO₃ remaining = 1 - 0.5 = 0.5 moles. Molecules remaining = 0.5 * 6.022 x 10²³ ≈ 3.01 x 10²³.