Problem 36 - Chemistry Matter and Change

Question

Identify each substance as a molecular compound or an ionic compound, and then calculate its molar mass. \begin{equation}a . {Sr}\left(\mathrm{NO}_{3}\right)_{2}\end{equation} \begin{equation}b. \left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}\end{equation} \begin{equation}\mathbf{c} . \mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\end{equation}

Step-by-Step Solution

Answer

a. Ionic, 211.63 g/mol. b. Ionic, 149.09 g/mol. c. Molecular, 342.30 g/mol.

Step 1: Classify Sr(NO3)2

Sr(NO3)2 is composed of a metal, strontium (Sr), and a polyatomic ion, nitrate (NO3-), indicating it is an ionic compound.

Step 2: Calculate the Molar Mass of Sr(NO3)2

The molar mass is calculated as follows: Sr = 87.62 g/mol; N = 14.01 g/mol; O = 16.00 g/mol. \[ \text{Molar mass of Sr(NO}_3\text{)}_2 = 87.62 + 2(14.01 + 3(16.00)) = 211.63 \text{ g/mol} \]

Step 3: Classify (NH4)3PO4

(NH4)3PO4 consists of the ammonium ion (NH4+) and phosphate ion (PO4-3), which are ions combined in the compound, making it an ionic compound.

Step 4: Calculate the Molar Mass of (NH4)3PO4

Calculate the molar mass: N = 14.01 g/mol; H = 1.008 g/mol; P = 30.97 g/mol; O = 16.00 g/mol. \[ \text{Molar mass of (NH}_4\text{)}_3\text{PO}_4 = 3(14.01 + 4(1.008)) + 30.97 + 4(16.00) = 149.09 \text{ g/mol} \]

Step 5: Classify C12H22O11

C12H22O11 is composed entirely of nonmetals (carbon, hydrogen, and oxygen), indicating it is a molecular compound.

Step 6: Calculate the Molar Mass of C12H22O11

The molar mass is calculated using: C = 12.01 g/mol; H = 1.008 g/mol; O = 16.00 g/mol. \[ \text{Molar mass of C}_{12}\text{H}_{22}\text{O}_{11} = 12(12.01) + 22(1.008) + 11(16.00) = 342.30 \text{ g/mol} \]

Key Concepts

Ionic Compounds

Ionic compounds form when metals and nonmetals join due to electron transfer. This transaction results in two oppositely charged ions: cations (positive) and anions (negative). These ions attract each other under the electromagnetic forces, forming a stable compound. For example, in the compound Sr(NO₃)₂, strontium creates a positive ion, while the nitrate forms a negative ion.

This strong ionic bonding typically creates compounds with high melting and boiling points. They dissolve easily in water, making them electrolytes capable of conducting electricity.

Understanding these features aids in recognizing ionic compounds and predicting their behaviors, which is crucial in chemical reactions and applications.

Molecular Compounds

Molecular compounds contain nonmetals bound together by covalent bonds, implying shared electrons rather than transferred ones. An example is C₁₂H₂₂O₁₁ (sucrose), a sugar composed only of nonmetals such as carbon, hydrogen, and oxygen.

These compounds usually exist in various states of matter—solid, liquid, or gas—and have comparatively lower melting and boiling points than ionic compounds.

Molecular compounds may not conduct electricity when dissolved in water, as they do not form ions. Their structure affects many characteristics, including aroma and taste, playing integral roles in biological systems and materials.

Chemical Classification

Chemical classification involves grouping substances based on shared properties into categories such as ionic or molecular compounds, metals, and nonmetals. Accurately classifying enables chemists to anticipate a substance’s behavior.

Simple rules-of-thumb can often help classify compounds. For instance:

• Ionic compounds generally consist of metals combined with nonmetals or polyatomic ions.
• Molecular compounds tend to include only nonmetals.

Recognizing these categories aids scientists and students in understanding reactivity, stability, and properties, streamlining learning about chemical phenomena extensively used in labs and industries.

Chemical Formulas

Chemical formulas provide valuable information about the composition and proportion of atoms in a compound. They use symbols from the periodic table and subscript numbers to indicate the number of each type of atom in the compound. For instance, in the chemical formula (NH₄)₃PO₄, (ammonium phosphate) the subscript indicates three ammonium ions for each phosphate ion.

Different types of formulas serve different purposes:

• Empirical formulas show the simplest atom ratio.
• Molecular formulas display the exact number of atoms of each type.
• Structural formulas reveal the arrangement of atoms.

Understanding these formulas is vital for carrying out stoichiometric calculations and determining molecular weight, which significantly benefits students analyzing compounds in academic studies and industry applications.