Identify the polyatomic ion and its charge in each compound (a) KNO2 K N O 2 (b) CaSO4 C a S O 4 (c) Mg(NO3)2

Explanation:

Polyatomic ions are ions (usually anions) that are made up of more than one atom. In order to determine the charge of anions, one can first identify the charge of the metal (which forms the cation) in a compound (which is usually easy to know) before predicting the charge of the anion. The charges are usually exchanged to form denominator of the other reacting atom/molecule, but if divisible, they are divided first before the exchange.

(a) The polyatomic ion in KNO₂ is NO₂ with the ionic equation below showing it's charge

KNO₂  ⇒ K⁺ + NO₂⁻

From the above, we can deduce that the charge of NO₂ is "1-"

(b) The polyatomic ion in CaSO₄ is SO₄ with the ionic equation below showing it's charge

CaSO₄ ⇒ Ca²⁺ + SO₄²⁻

The charge of SO₄ from the above equation is "2-"

(c) The polyatomic ion in Mg(NO₃)₂ is NO₃ with the ionic equation below showing it's charge

Mg(NO₃)₂   ⇒ Mg²⁺  +  NO₃⁻

From the above equation, it can be deduced that the charge of NO₃ is "1-"

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