Q108P - Chemistry: Molecular Nature Of Matter And Change

Question

Calculate the molecular(formula) mass of each compound:

(a)   Dinitrogenpentaoxide;

(b)   Lead(II) nitrate;

(c)   Calcium peroxide.

Step-by-Step Solution

Answer

(a)   the molecular mass of the Dinitrogen pentaoxide is 108.00 amu.

(b)   The Molecular mass of the Lead(II) nitrates is331.20 amu.

(c)    the molecular mass of the Calcium peroxide is72.08 amu.

Step 1: To calculate the molecular mass for dinitrogen pentaoxide

The molecule given is a covalent compound. Therefore, the number of atoms depends on the prefix.

There are 2 nitrogen atoms as the prefix di- suggest, and there are 5 oxygen atoms as the prefix Penta- suggests. So, the molecular formula of dinitrogen pentaoxide is${\mathbf{N}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{5}}$

List the atomic mass of each of the elements present.

$\mathbf{N}\mathbf{=}\mathbf{14}\mathbf{.}\mathbf{00}\mathbf{u}\mathbf{ }\mathbf{O}\mathbf{=}\mathbf{16}\mathbf{.}\mathbf{00}\mathbf{u}$

Then add the atomic masses of the constituent atoms.

Molecular mass$\mathbf{=}\mathbf{2}\mathbf{\times }\mathbf{atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{N}\mathbf{+}\mathbf{5}\mathbf{ }\mathbf{atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{O}$

$\mathbf{=}\mathbf{2}\left(14.00\right)\mathbf{+}\mathbf{5}\left(16.00\right)$

Molecular massamu$\mathbf{=}\mathbf{108}\mathbf{.}\mathbf{00}\mathbf{ }\mathbf{amu}\mathbf{ }\mathbf{amu}$.

Step 2: To calculate the molecular mass for Lead (II) nitrate

The molecule is an ionic compound. The number of atoms depends on the charge of each ion.

Lead has a charge of +2and nitrate is a polyatomic anion with the formula${\mathbf{NO}}_{\mathbf{3}}^{\mathbf{-}}$.

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

${\mathbf{Pb}}^{\mathbf{2}\mathbf{+}}{\mathbf{NO}}_{\mathbf{3}}^{\mathbf{-}}\mathbf{\to }\mathbf{Pb}{\left({\mathrm{NO}}_3\right)}_{\mathbf{2}}$

List the atomic mass of each of the elements present.

$\mathbf{Pb}\mathbf{=}\mathbf{207}\mathbf{.}\mathbf{20}\mathbf{u}\mathbf{,}\mathbf{ }\mathbf{N}\mathbf{=}\mathbf{14}\mathbf{.}\mathbf{01}\mathbf{u}\mathbf{,}\mathbf{ }\mathbf{O}\mathbf{=}\mathbf{16}\mathbf{.}\mathbf{00}\mathbf{u}$

Then add the atomic masses of the constituent atoms.

Molecular mass

$$\begin{gathered} \mathbf{=}\mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{Pb}\mathbf{+}\mathbf{2}\mathbf{\times }\mathbf{atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{N}\mathbf{+}\mathbf{6}\mathbf{\times }\mathbf{atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{O} \\ \mathbf{=}\mathbf{207}\mathbf{.}\mathbf{20}\mathbf{+}\mathbf{2}\mathbf{\times }\mathbf{14}\mathbf{.}\mathbf{00}\mathbf{+}\mathbf{6}\mathbf{\times }\mathbf{16}\mathbf{.}\mathbf{00} \end{gathered}$$

Molecular massamu$\mathbf{=}\mathbf{331}\mathbf{.}\mathbf{20}\mathbf{ }\mathbf{amu}\mathbf{ }\mathbf{amu}$.

Step 3: To calculate the molecular mass of Calcium peroxide

It is an ionic compound. The number of atoms depends on the charge of each ion. Calcium is a group II element with a charge of +2. Peroxide is a polyatomic ion with the formula${\mathbf{O}}_{\mathbf{2}}^{\mathbf{2}\mathbf{-}}$.

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

${\mathbf{Ca}}^{\mathbf{2}\mathbf{+}}{\mathbf{O}}_{\mathbf{2}}^{\mathbf{-}\mathbf{2}}\mathbf{\to }{\mathbf{Ca}}_{\mathbf{2}}{\left(O_2\right)}_{\mathbf{2}}\stackrel{\mathbf{lowest}\mathbf{ }\mathbf{tems}}{\mathbf{\to }}{\mathbf{CaO}}_{\mathbf{2}}$

List the atomic mass of each of the elements present.

$\mathbf{Ca}\mathbf{=}\mathbf{40}\mathbf{.}\mathbf{08}\mathbf{ }\mathbf{u}\mathbf{,}\mathbf{ }\mathbf{O}\mathbf{=}\mathbf{16}\mathbf{.}\mathbf{00}\mathbf{u}$

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

Molecular mass$\mathbf{Atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{Ca}\mathbf{+}\mathbf{2}\mathbf{\times }\mathbf{atomic}\mathbf{ }\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{O}$

$\mathbf{=}\mathbf{40}\mathbf{.}\mathbf{08}\mathbf{+}\mathbf{2}\left(16.00\right)$

Molecular mass$\mathbf{=}\mathbf{72}\mathbf{.}\mathbf{08}\mathbf{ }\mathbf{amu}$.