2.4.1 Describing Chemical Formulas

Question 1

Describing Chemical Formulas

Answer 1

• Atoms, molecules, ions, ionic compounds, and covalent solids are all different units of matter.
• Chemical formulas are a “chemical shorthand” to quickly and accurately identify compounds.
• Chemical formulas can be represented graphically in many different ways.

Question 2

atoms, molecules, ions

Answer 2

• Atoms are the smallest units of an element.
• Molecules are well-defined groups of atoms that are
  chemically bonded together.
• Ions are charged particles that are obtained from atoms by adding or removing electrons. These are also polyatomic ions.
• Ionic compounds and covalent solids are three-dimensional repeating structures. Ionic compounds are composed of positive ions (cations) and negative ions (anions) in a set ratio. Covalent solids are composed of covalently bonded atoms in a fixed ratio

Question 3

chemical formulas

Answer 3

• Chemical formulas are a “chemical shorthand” to quickly and accurately identify compounds.
• Molecular formulas represent the actual number of atoms of each element in a molecule. For example, the molecular formula for benzene is C6H6.
• Empirical formulas represent the simplest integer ratio of the elements in a compound. For example, the empirical formula for benzene is CH. The empirical formula does not always uniquely identify a molecule—CH could be benzene (C6H6), but it could also be ethyne (C2H2).
• Chemical formulas can be represented graphically in many different ways.
• A line drawing allows chemists to see how the individual atoms connect together.
• A shorthand is used by organic chemists to quickly write common molecules.
• A ball and stick representation closely resembles the actual geometry of a molecule.
• A space filling representation shows what a molecule would look like magnified.

Question 4

NH4+ is an example of

Answer 4

a cation.

Question 5

Which of the following statements is false?

Answer 5

Butane and 2-methylpropane form lattices.
- The structural formulas above show that each substance has a molecular structure, not a lattice structure. Lattices are potentially infinite arrays of well-defined ratios, but undefined numbers, of types of ions or atoms forming either an ionic salt or a covalent lattice. There is no clear way to identify a molecule of these types of substances.

Question 6

For sodium chloride, an ionic compound, which of the following can you NOT do?

Answer 6

Write a molecular formula.
- Sodium chloride is an ionic salt, which forms a lattice. There is no defined molecular boundary. The ratio of one element to the other can be determined, but not the exact number of each element. Therefore, a molecular formula cannot be written.

Question 7

A molecule of caffeine consists of 8 carbon atoms, 10 hydrogen atoms, 4 nitrogen atoms, and 2 oxygen atoms. Which of the following represents the empirical formula for caffeine?

Answer 7

C4H5N2O

Question 8

Sulfate (SO42− ) is

Answer 8

a polyatomic anion.

Question 9

_____________________ are discrete packages of atoms connected together in well-defined numbers and ratios with no overall charge.

Answer 9

Molecules

Question 10

Which of the following is a molecule?

Answer 10

CO2

Question 11

C2H2 is a chemical formula for acetylene. Which type of chemical formula is it?

Answer 11

A molecular formula

Question 12

Which of the following statements is false?

Answer 12

A molecular formula uniquely describes a molecule

Question 13

The following is a ball-and-stick drawing of the compound cis 1,2-dichloroethene.

What is the empirical formula for 1,2-dichloroethene?

Answer 13

CHCl