To determine the formula of magnesium hydrogen carbonate, we need to recognize the chemical composition of magnesium, a metal, and hydrogen carbonate, an anion.
Magnesium has the chemical symbol \( \text{Mg} \).
Hydrogen carbonate, also known as bicarbonate, has the chemical formula \( \text{HCO}_3^- \).
Magnesium hydrogen carbonate forms when magnesium ions (Mg²⁺) combine with hydrogen carbonate ions (\(\text{HCO}_3^-\)). Since magnesium has a +2 charge and each hydrogen carbonate anion has a -1 charge, we need two hydrogen carbonate ions to balance the charge of one magnesium ion:
[tex]\[
\text{Mg}^{2+} + 2 \text{HCO}_3^- \rightarrow \text{Mg}(\text{HCO}_3)_2
\][/tex]
Therefore, the correct formula for magnesium hydrogen carbonate is:
[tex]\[
\text{Mg}(\text{HCO}_3)_2
\][/tex]
Among the options provided:
- \( \text{Mn}(\text{HCO}_3)_2 \) is incorrect, as it uses manganese (Mn) instead of magnesium (Mg).
- \( \text{MgCO}_3 \) is incorrect, as this is the formula for magnesium carbonate, not magnesium hydrogen carbonate.
- \( \text{MgClO} \) is incorrect, as it does not represent hydrogen carbonate but rather contains hypochlorite (ClO).
Thus, the correct choice is:
[tex]\[
\boxed{4}
\][/tex]
