Question

Write the chemical formula of the combination of the following ions:
(i) Chloride and potassium
(ii) Lead (IV) and Sulphate
(iii)Phosphate and hydrogen

Answer 1

Hint: The compound formed when the elements combine. The molecular formula for such compounds depends on the elements taking part in the formation of a chemical compound. The molecular formula is written such that the symbols of the electropositive element are written first followed by the electronegative element. The subscript is added such that the charge on the compound remains maintained.

Compete for step by step answer:
The number of bonds formed between the element or group of elements depends on the property of an atom or molecule to gain or lose electrons. The name is written such that the charge on the compound is maintained.
Following are the steps to follow to determine the molecular formula of a compound:
Step 1) Determine the symbol for each element in the compound to form
Step2) Identify whether the compound is inorganic or organic. If organic then the chemical structure is written in such a way that carbon is written first then hydrogen followed by the other elements present in the compound. For inorganic compounds, the elements which are lower are written first (or electropositive ) followed by the higher (electronegative) element.
Step 3) write the appropriate subscript so that the number of atoms forms a stable compound. Let's have a look at the given ions:
(i) Chloride and potassium:
The elemental symbol for chloride is $\text{ Cl }$ and for potassium is $\text{ K }$. The general molecular formula can be written as $\text{ }{{\text{K}}_{\text{x}}}\text{C}{{\text{l}}_{\text{y}}}\text{ }$ where x and y are subscript of $\text{ K }$and $\text{ Cl }$ respectively. Since both potassium and chloride have equal charge 1. The molecular formula is $\text{ KCl }$ .
(ii) Lead (IV) and Sulphate:
The elemental symbol for lead is $\text{ Pb }$ and molecular formulas for sulphate are $\text{ SO}_{4}^{2-}\text{ }$. The general molecular formula can be written as $\text{ P}{{\text{b}}_{\text{x}}}{{\left( \text{S}{{\text{O}}_{\text{4}}} \right)}_{\text{y}}}\text{ }$ where x and y are subscript of $\text{ Pb }$ and $\text{ SO}_{4}^{2-}\text{ }$ respectively. Lead has a $\text{ +4 }$ charge and sulphate ion exists in the $\text{ }-2\text{ }$ change. Thus the molecular formulas are $\text{ Pb(S}{{\text{O}}_{\text{4}}}{{\text{)}}_{\text{2}}}\text{ }$ .
(iii)Phosphate and hydrogen:
The elemental symbol for hydrogen is $\text{ H }$ and molecular formulas for phosphate are \[\text{ PO}_{4}^{3-}\text{ }\]. The general molecular formula can be written as $\text{ }{{\text{H}}_{\text{x}}}{{\left( \text{P}{{\text{O}}_{\text{4}}} \right)}_{\text{y}}}\text{ }$ where x and y are subscript of $\text{ H }$and \[\text{ PO}_{4}^{3-}\text{ }\] respectively. Hydrogen has a $\text{ +1 }$ charge and phosphate ion exists in the $\text{ }-3\text{ }$ change. Thus the molecular formulas are $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }$ .

Note: The molecular structure can be determined by crossing the valency of the element or group of elements. This is a little trick to get the molecular formula. For example, lead (IV) and sulphate structure is determined as follows,
$\text{ }\begin{matrix}
\text{Element / Molecule} & {} & \text{Valency} \\
\text{Pb} & \searrow & +4 \\
\text{SO}_{\text{4}}^{2-} & \nearrow & -2 \\
\end{matrix}\text{ }\Rightarrow \text{ P}{{\text{b}}_{\text{2}}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{\text{4}}}\text{ }\Rightarrow \text{Pb(S}{{\text{O}}_{\text{4}}}{{\text{)}}_{2}}\text{ }$
Thus structure of the molecule can be obtained by crossing the valencies.