What is [H3O+] in a solution obtained by dissolving 215 mL HCl(g), measured at 23 degrees C and 757 mmHg, in 4.30 L of aqueous solution?

Answer 1

Use PV = nRT and calculate n = number of moles HCl. Don't forget to change mL to liters and T to Kelvin.
That will give you moles. The molarity = mols/L solution = moles/4.30 = ?? molarity of the final solution.