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Ch.9 - Chemical Bonding I: The Lewis Model
Chapter 9, Problem 91

Formic acid is responsible for the sting of ant bites. By mass, formic acid is 26.10% C, 4.38% H, and 69.52% O. The molar mass of formic acid is 46.02 g/mol. Determine the molecular formula of formic acid and draw its Lewis structure.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Empirical Formula

The empirical formula represents the simplest whole-number ratio of the elements in a compound. To determine it, the percentage composition of each element is converted to moles, and then the mole ratios are simplified. For formic acid, the empirical formula can be derived from its mass percentages of carbon, hydrogen, and oxygen.
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Empirical vs Molecular Formula

Molecular Formula

The molecular formula indicates the actual number of atoms of each element in a molecule of a compound. It can be derived from the empirical formula by comparing the molar mass of the compound to the molar mass of the empirical formula. In the case of formic acid, knowing its molar mass allows us to determine how many times the empirical formula fits into the molecular formula.
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Determining Molecular Formulas

Lewis Structure

A Lewis structure is a diagram that represents the bonding between atoms in a molecule and the lone pairs of electrons. It helps visualize the arrangement of electrons and the connectivity of atoms. For formic acid, drawing the Lewis structure involves placing the correct number of valence electrons around the carbon, hydrogen, and oxygen atoms to satisfy the octet rule where applicable.
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Related Practice
Open Question
Does each compound contain both ionic and covalent bonds? Write ionic Lewis structures for each of them, including the covalent structure for the ion in brackets. Write resonance structures if necessary for the following: b. NH4Cl, d. Sr(CN)2, a. RbIO2, c. KOH.
Textbook Question

Carbon ring structures are common in organic chemistry. Draw a Lewis structure for each carbon ring structure, including any necessary resonance structures. a. C4H8 b. C4H4 c. C6H12 d. C6H6

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Textbook Question

Amino acids are the building blocks of proteins. The simplest amino acid is glycine (H2NCH2COOH). Draw a Lewis structure for glycine. (Hint: The central atoms in the skeletal structure are nitrogen and the two carbon atoms. Each oxygen atom is bonded directly to the right-most carbon atom.)

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Textbook Question

Diazomethane is a highly poisonous, explosive compound because it readily evolves N2. Diazomethane has the following composition by mass: 28.57% C; 4.80% H; and 66.64% N. The molar mass of diazomethane is 42.04 g/mol. Find the molecular formula of diazomethane, draw its Lewis structure, and assign formal charges to each atom. Why is diazomethane not very stable? Explain.

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Textbook Question

The reaction of Fe2O3(s) with Al(s) to form Al2O3(s) and Fe(s) is called the thermite reaction and is highly exothermic. What role does lattice energy play in the exothermicity of the reaction?

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Textbook Question

NaCl has a lattice energy of -787 kJ/mol. Consider a hypothetical salt XY. X3+ has the same radius of Na+ and Y3- has the same radius as Cl-. Estimate the lattice energy of XY.

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