Answer:
6-
[tex]{ \sf{XCO _{3} }}[/tex]
because X has only one positive charge.
7-
[tex]{ \sf{increases}}[/tex]
because there is increase in hydroxyl ions which decrease the acidity or hydrogen strength of hydrochloric acid.
8-
[tex]{ \sf{Ca(NO _{3} ) _{2} }}[/tex]
Calcium has a valency of 2 and nitrate ion radicle has valency of 1