What was the significance of the ""dashes"" or blank spots on mendeleev’s original periodic table?

What is the formula that this ionic compounds could form sr2+p3-o2-

Idon’t know how to solve this question

Astudent completes a titration by adding 12.0 milliliters of naoh(aq) of unknown concentration to 16.0 milliliters of 0.15 m hcl(aq). what is the molar concentration of the naoh(aq)? 1)5.0 m 2)0.20 m 3)0.11 m 4)1.1 m