How can I balance these equations using oxidation numbers?

(a)
unbalanced equation : K(+) + Ca → K + Ca(2+)

For K(+) → K, the oxidation number of K decreases from +1 to 0
Decrease in oxidation number = 1

For Ca → Ca(2+), the oxidation number of Ca increases from 0 to +2.
Increase in oxidation number = 2

There should be no net change in oxidation number.
The change of K(+) → K is multiplied by 2.
The change of Ca→ Ca(2+) is multiplied by 1.

The balanced equation : 2K(+) + Ca → 2K + Ca(2+)


(b)
unbalanced equation : Mg + Zn(2+) → Mg(2+) + Zn

For Zn(2+) → Zn, the oxidation number of Zn decreases from +2 to 0
Decrease in oxidation number = 2

For Mg → Mg(2+), the oxidation number of Mg increases from 0 to +2.
Increase in oxidation number = 2

There should be no net change in oxidation number.
The change of Zn(2+) → Zn is multiplied by 1.
The change of Mg → Mg(2+) is also multiplied by 1.

The balanced equation : Mg + Zn(2+) → Mg(2+) + Zn


(c)
unbalanced equation : Ca + Cr(3+) → Ca(2+) + Cr

For Cr(3+) → Cr, the oxidation number of Cr decreases from +3 to 0
Decrease in oxidation number = 3

For Ca → Ca(2+), the oxidation number of Ca increases from 0 to +2.
Increase in oxidation number = 2

There should be no net change in oxidation number.
The change of Cr(3+) → Cr is multiplied by 2.
The change of Ca → Ca(2+) is multiplied by 3.

The balanced equation : 3Ca + 2Cr(3+) → 3Ca(2+) + 2Cr


(d)
unbalanced equation : Na + O2 → Na(+) + O(2-)

For O2 → 2O(2-), the oxidation number of O decreases from 0 to -2
Decrease in oxidation number = 2 × 2 = 4

For Na → Na(+), the oxidation number of Na increases from 0 to +1.
Increase in oxidation number = 1

There should be no net change in oxidation number.
The change of O2 → 2O(2-) is multiplied by 1.
The change of Na → Na(+) is multiplied by 4.

The balanced equation : 4Na + O2 → 4Na(+) + 2O(2-)

"here are the equation:
1) 2K(+) + Ca → 2K + Ca(2+)
2) Mg + Zn(2+) → Mg(2+) + Zn
3) 3Ca + 2Cr(3+) → 3Ca(2+) + 2Cr
4) 4Na + O2 → 4Na(+) + 2O(2-)

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