Which of these properties is the best one to use for identification of an element ?

Calculate the h3o+ concentration in a solution of acetic acid if the concentration of molecular acetic acid present at equilibrium is 9.97x10^-3 m and k for the dissociation is 1.86x10^-5. ch3cooh(aq)+h2o(> h3o^+(aq)+ch3coo^-(aq)

Which other elements contain the same number of outer electrons as sodium

Name the intersection of plane bpq and plane cpq.