Why does Silica have the formula ${\mathrm{SiO}}_2$?

Decide whether this formula is correct. If it is not correct, write it correctly.

${\mathrm{HBr}}_2$

Decide whether this formula is correct. If it is not correct, write it correctly.

$\mathrm{ClNa}$

Decide whether this formula is correct. If it is not correct, write it correctly.

${\mathrm{Cl}}_3\mathrm{Ca}$

Decide whether this formula is correct. If it is not correct, write it correctly.

${\mathrm{Ba}}_2\mathrm{O}$

Balance the following equation:

$\mathrm{Na}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to \mathrm{NaCl}\left(\mathrm{s}\right)$