244) The number of OH⁻ ions in 1 mL of solution having pH = 4 is:

245) For preparing a buffer solution of pH 6 by mixing sodium acetate and acetic acid, the ratio of the concentration of salt and acid should be: (K^a = 10⁻⁵)

246) 0.15 mol of pyridinium chloride has added to 500 cc of 0.2M pyridine solution (K_b for pyridine is 1.5 x 10⁻⁹M). pH is:

247) M(OH)_x has K_sp = 4 x 10⁻¹² and solubility 10⁻⁴. x is:

248) To a mixture of NH₄Cl and NH₃ in water, a further amount of NH₄Cl is added. The pH of the mixture is: