Q109P - Chemistry: Molecular Nature Of Matter And Change

Question

Calculate the molecular(formula) mass of each compound:

a)    Iron(II) acetate tetrahydrate;

b)    Sulfur tetrachloride;

c)    Potassium permanganate.

Step-by-Step Solution

Answer

(a)   the molecular mass of the Iron(II) acetate tetrahydrate is 246.03 amu.

(b)   The Molecular mass of the Sulfur  tetrachlorideis173.87 amu.

(c)   the molecular mass of the Potassium permanganateis158.04 amu.

Step 1: To calculate the molecular mass for iron(II) acetate tetrahydrate

It is an anionic compound. The number of atoms depends on the charge of each ion. Iron has a charge of +2. Acetate is a polyatomic anion with the formula${\mathbf{CH}}_{\mathbf{3}}{\mathbf{COO}}^{\mathbf{-}}$.

The charges of each ion will just interchange to give the molecular formula.

$$\begin{gathered} {\mathbf{Fe}}^{\mathbf{2}\mathbf{+}}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{COO}}^{\mathbf{-}}\mathbf{\to }\mathbf{Fe}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_{\mathbf{2}} \\ \left(12.01+3\left(1.01\right)+12.01+16.00\right)\mathbf{+}\mathbf{4}\left(2\left(1.01\right)+16.00\right) \end{gathered}$$

The prefix tetra suggests that it has 4 water molecules,

So, we have the formula$\mathbf{Fe}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_{\mathbf{2}}\mathbf{.}\mathbf{4}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$

List the atomic mass of each of the elements present.

$\mathbf{Fe}\mathbf{=}\mathbf{55}\mathbf{.}\mathbf{85}\mathbf{ }\mathbf{u}\mathbf{ }\mathbf{C}\mathbf{=}\mathbf{12}\mathbf{.}\mathbf{01}\mathbf{u}\mathbf{ }\mathbf{H}\mathbf{=}\mathbf{1}\mathbf{.}\mathbf{01}\mathbf{u}\mathbf{ }\mathbf{O}\mathbf{=}\mathbf{16}\mathbf{.}\mathbf{00}\mathbf{ }\mathbf{u}$

Then add the atomic masses of the constituent atoms.

Molecular mass

$$\begin{gathered} \mathbf{=}\mathbf{Fe}\mathbf{+}\mathbf{2}\left(C+3H+C+O+O\right)\mathbf{+}\mathbf{4}\left(2H+O\right) \\ \mathbf{=}\mathbf{55}\mathbf{.}\mathbf{85}\mathbf{+}\mathbf{2}\left(12.01+3\left(1.01\right)+12.01+16.00+16.00\right)\mathbf{+}\mathbf{4}\left(2\left(1.01\right)+16.00\right) \end{gathered}$$

Molecular mass$\mathbf{=}\mathbf{246}\mathbf{.}\mathbf{03}\mathbf{ }\mathbf{amu}$.

Step 2: To calculate the molecular mass for sulfur tetrachloride

It is a covalent compound. The number of atoms depends on the prefix of each atom. There is one sulfur atom because it has no prefix and there are four chlorine atoms because of the prefix tetra-.

So, the molecular formula is${\mathbf{SCI}}_{\mathbf{4}}$.

List the atomic mass of each of the elements present.

$\mathbf{S}\mathbf{=}\mathbf{32}\mathbf{.}\mathbf{07}\mathbf{u}\mathbf{ }\mathbf{CI}\mathbf{=}\mathbf{35}\mathbf{.}\mathbf{45}\mathbf{u}$

Then add the atomic masses of the constituent atoms to find out the molecular mass.

Molecular mass

$$\begin{gathered} \mathbf{=}\mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{S}\mathbf{+}\mathbf{4}\mathbf{\times }\mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{CI} \\ \mathbf{=}\mathbf{32}\mathbf{.}\mathbf{07}\mathbf{+}\mathbf{4}\left(35.45\right) \end{gathered}$$

Molecular mass$\mathbf{=}\mathbf{173}\mathbf{.}\mathbf{87}\mathbf{ }\mathbf{amu}$.

Step 3: To calculate the molecular mass for potassium permanganate

It is an ionic compound. The number of atoms depends on the charge of each ion. Potassium is a group 1 element so it has a charge of +1.

Permanganate is a polyatomic ion with the formula${\mathbf{MnO}}_{\mathbf{4}}^{\mathbf{-}}$.

The Charges of each ion will just interchange as subscripts of its molecular formula.

${\mathbf{K}}^{\mathbf{+}}{\mathbf{MnO}}_{\mathbf{4}}^{\mathbf{-}}\mathbf{\to }{\mathbf{KMnO}}_{\mathbf{4}}$

First of all, list the atomic mass of each element present.

$\mathbf{K}\mathbf{=}\mathbf{39}\mathbf{.}\mathbf{10}\mathbf{u}\mathbf{,}\mathbf{ }\mathbf{Mn}\mathbf{=}\mathbf{54}\mathbf{.}\mathbf{94}\mathbf{u}\mathbf{,}\mathbf{ }\mathbf{O}\mathbf{=}\mathbf{16}\mathbf{.}\mathbf{00}\mathbf{u}$

Then add the atomic masses of the constituent atoms to get the molecular mass.

Molecular mass

$$\begin{gathered} \mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{K}\mathbf{+}\mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{Mn}\mathbf{+}\mathbf{4}\mathbf{\times }\mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{O} \\ \mathbf{=}\mathbf{39}\mathbf{.}\mathbf{10}\mathbf{+}\mathbf{54}\mathbf{.}\mathbf{94}\mathbf{+}\mathbf{4}\left(16.00\right) \end{gathered}$$

Molecular mass$\mathbf{=}\mathbf{158}\mathbf{.}\mathbf{04}\mathbf{ }\mathbf{amu}$.