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H2
OH^-
Mg^2+
CO3^2-
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For H2:
Since hydrogen (H) is in its elemental form, its oxidation number is always 0.
For OH^-:
The overall charge of the hydroxide ion is -1. Oxygen (O) usually has an oxidation number of -2 when it appears in compounds, so we assign -2 to oxygen. Since the hydroxide ion has a charge of -1, the oxidation number of hydrogen (H) can be determined by the following formula: -1 (-2) = +1. Therefore, the oxidation number of hydrogen in this case is +1.
For Mg^2+:
The overall charge of magnesium (Mg) ion is +2. In its compounds, the oxidation number of magnesium is always +2. Therefore, the oxidation number of magnesium in this case is +2.
For CO3^2-:
The overall charge of the carbonate ion is -2. Oxygen (O) usually has an oxidation number of -2 when it appears in compounds, so we assign -2 to each oxygen atom. The sum of the oxidation numbers of all the elements in the ion must be equal to the charge of the ion. Let's assign the oxidation number of carbon (C) as x. Applying the sum of oxidation numbers equation: x + 3(-2) = -2. Simplifying, we get x - 6 = -2. Solving for x, we find that the oxidation number of carbon is +4.
In summary, the oxidation numbers for each element are as follows:
- H2: H = 0
- OH^-: H = +1, O = -2
- Mg^2+: Mg = +2
- CO3^2-: C = +4, O = -2