Question

Calculate the molecular (formula) mass of each compound: (a) dinitrogen pentoxide; (b) lead(II) nitrate; (c) calcium peroxide.

Short answer

Dinitrogen pentoxide: 108.02, Lead(II) nitrate: 331.22, Calcium peroxide: 72.08.

Step-by-step solution

Identify the elements and their atomic masses

Look up the atomic masses of each element involved in the compounds. Use the periodic table for reference. They are: Nitrogen (N) - 14.01, Oxygen (O) - 16.00, Lead (Pb) - 207.2, Calcium (Ca) - 40.08.

Write the molecular formula for each compound

Dinitrogen pentoxide - \( N_2O_5 \), Lead(II) nitrate - \( Pb(NO_3)_2 \), Calcium peroxide - \( CaO_2 \).

Calculate the total atomic mass for dinitrogen pentoxide

Dinitrogen pentoxide has 2 nitrogen atoms and 5 oxygen atoms. Compute: Total mass = (2 × 14.01) + (5 × 16.00) Total mass = 28.02 + 80.00 Total mass = 108.02

Calculate the total atomic mass for lead(II) nitrate

Lead(II) nitrate has 1 lead atom, 2 nitrogen atoms, and 6 oxygen atoms (since there are 2 nitrate groups). Compute: Total mass = 207.2 + (2 × 14.01) + (6 × 16.00) Total mass = 207.2 + 28.02 + 96.00 Total mass = 331.22

Calculate the total atomic mass for calcium peroxide

Calcium peroxide has 1 calcium atom and 2 oxygen atoms. Compute: Total mass = 40.08 + (2 × 16.00) Total mass = 40.08 + 32.00 Total mass = 72.08

Key concepts

Atomic Mass

Understanding atomic mass is crucial for calculating molecular masses. Atomic mass refers to the mass of a single atom of a chemical element. It includes the mass of the protons, neutrons, and electrons, but since electrons have very little mass, it's primarily the protons and neutrons. For example:

• Nitrogen (N) has an atomic mass of 14.01.
• Oxygen (O) has an atomic mass of 16.00.
• Lead (Pb) has an atomic mass of 207.2.
• Calcium (Ca) has an atomic mass of 40.08.

You can find these values in the periodic table. Each element's atomic mass is usually found below the element symbol. When calculating the molecular mass, you combine the atomic masses of all atoms in the molecule.

Molecular Formula

A molecular formula shows the number and type of atoms in a molecule. It helps us understand the composition of the compound. For instance:

• Dinitrogen pentoxide has the molecular formula \(\text{N}_2\text{O}_5\).
• Lead(II) nitrate has the molecular formula \(\text{Pb(NO}_3\text{)}_2\).
• Calcium peroxide has the molecular formula \(\text{CaO}_2\).

This notation is important because it reveals the exact number of each kind of atom in the compound. Each element’s atomic symbol is followed by a subscript denoting the number of atoms of that element in one molecule.

Compound

A compound consists of atoms of two or more different elements chemically bonded together. These compounds have distinct properties and chemical behaviors. For example, let's consider:

• Dinitrogen Pentoxide (\(\text{N}_2\text{O}_5\)): composed of nitrogen and oxygen in a 2:5 ratio.
• Lead(II) Nitrate (\(\text{Pb(NO}_3\text{)}_2\)): composed of lead, nitrogen, and oxygen. The \(\text{NO}_3-\) is a nitrate group, and there are 2 of them.
• Calcium Peroxide (\(\text{CaO}_2\)): composed of calcium and oxygen in a 1:2 ratio.

To form a compound, the elements must go through chemical reactions where their atoms share, gain, or lose electrons, leading to the release or absorption of energy.