A chemical formula represents a substance using chemical symbols and numerical subscripts, indicating the types of atoms and their proportions.
What are the two main types of chemical formulas?
The two main types of chemical formulas are empirical and molecular formulas.
What is an empirical formula?
An empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.
Does the empirical formula show the actual number of atoms in a molecule?
No, the empirical formula does not show the actual number of atoms in a molecule; it only shows the simplest ratio.
How do you find the empirical formula?
To find the empirical formula, convert mass to moles, establish the mole ratio, and use the mole ratio as subscripts.
What is a molecular formula?
A molecular formula shows the precise number of each type of atom in a molecule of a compound.
How does a molecular formula differ from an empirical formula?
A molecular formula shows the actual number of atoms in a molecule, while an empirical formula shows the simplest ratio.
How do you find the molecular formula?
To find the molecular formula, determine the empirical formula, calculate its mass, and multiply subscripts by the ratio of molecular mass to empirical mass.
What does the subscript in a chemical formula represent?
A subscript in a chemical formula indicates the number of atoms of each element in the molecule or formula unit.
What is the general rule for writing a chemical formula?
The general rule is to write the symbol for each element and use subscripts to indicate the number of atoms.
What is the formula for water?
The formula for water is H₂O.
What is the formula for carbon dioxide?
The formula for carbon dioxide is CO₂.
What is the formula for sodium chloride?
The formula for sodium chloride is NaCl.
What is the formula for magnesium chloride?
The formula for magnesium chloride is MgCl₂.
What type of compound is represented by NaCl?
NaCl represents an ionic compound.
What type of compound is represented by H₂O?
H₂O represents a covalent compound.
If a compound has an empirical formula of CH₂ and a molecular mass of 56, what is its molecular formula?
The molecular formula is C₄H₈. (CH₂ has a mass of 14. **56/14 = 4**, so C₄H₈.)
What is a cation?
A cation is a positively charged ion.
What is an anion?
An anion is a negatively charged ion.
What is the role of oxidation numbers in writing chemical formulas?
Oxidation numbers help balance charges in ionic compounds, especially with polyatomic ions, to ensure a neutral compound.
Explanation Questions
Explain the difference between an empirical and a molecular formula using an example.
An empirical formula is the simplest ratio of atoms in a compound, such as CH₂O for glucose, while a molecular formula is the actual number of atoms, such as C₆H₁₂O₆ for glucose.
Describe the steps involved in determining the empirical formula of a compound from mass percentages of elements.
To determine the empirical formula from mass percentages, first, convert the mass of each element to moles. Next, find the mole ratio among the elements. Finally, use the mole ratios as subscripts in the empirical formula.
How is the molecular formula of a compound related to its empirical formula?
The molecular formula is an integer multiple of the empirical formula, where the molecular formula equals (Empirical formula)ₙ.
Explain the ‘cross-over’ method for writing chemical formulas for ionic compounds.
The cross-over method involves using the numerical charge of one ion as the subscript for the other ion, ensuring the net charge is zero. For example, with magnesium (Mg²⁺) and chlorine (Cl⁻), the 2 becomes the subscript for chlorine, resulting in MgCl₂.
Describe how to use oxidation numbers to write the chemical formula for a compound that contains polyatomic ions.
Use oxidation numbers to ensure the total positive charge equals the total negative charge. For example, ammonium nitrate, (NH₄NO₃), has +1 from NH₄⁺ and -1 from NO₃⁻ resulting in a net charge of 0.
Why is it important to balance the charges in ionic compounds when writing their chemical formulas?
Balancing charges is crucial to ensure the compound is neutral, as ionic compounds are formed by electrostatic attraction between positive and negative charges which must equal zero.
Explain why the subscript ‘1’ is omitted in chemical formulas, using an example.
The subscript ‘1’ is omitted for simplicity; for example, in water (H₂O), the ‘1’ for the oxygen atom is not written and implies there is one oxygen atom present.
Describe the similarities and differences between the steps used to write chemical formulas for covalent compounds and ionic compounds.
Both use element symbols and subscripts. However, covalent compounds show the exact number of atoms, while ionic compounds use the cross-over method to balance charges.
How does the concept of moles relate to determining empirical formulas?
Moles are used to convert masses of elements to a common unit, which allows for determining the simplest ratio of atoms, which can then be used to create the empirical formula.
If you have a compound made up of carbon, hydrogen, and oxygen with an empirical formula of CH₂O and a molar mass of 180 g/mol, how would you go about finding its molecular formula?
1. Find the mass of the empirical formula CH₂O: **C = 12, H = 2, O = 16 → 12 + 2 + 16 = 30 g/mol**. 2. Divide the molar mass of the compound by the mass of the empirical formula: **180/30 = 6**. 3. Multiply the subscripts in the empirical formula (CH₂O) by **6** to find the molecular formula: **C₆H₁₂O₆**.