Chemical Quantities

Definition

A mole is a chemical unit of measurement, used to signify the amount of a substance within the International System of Units

Avogadro's Constant

One mole of a given substance is equal to: 6.022 × 10²³, This is known as Avogadro's Constant

Dimensional Analysis and Conversions

Conversions between the number of matter in a substance and mass can be calculated by using dimensional analysis.

Definition

Molar mass is the amount of substance that is represented within 1 mole of a given compound. The units are measured in g/mol.

The molar mass of a given element is equivalent to its atomic mass. For example:

Oxygen's Atomic mass: 15.999 amu
Oxygen's Molar mass: 15.999g/mol

Calculating Molar Mass

Molar mass is calculated through the addition of all monoatomic elements within a compound or series of compounds.

Putting it Together

By Understanding the concepts of the mole, avogadro's constant, and molar mass, you can solve problems utilizing each of their individual components. To do so, follow this simple diagram to help guide you.

Definition

The percent composition of a compound lists the mass of each element as a percentage.

Example: CO2
72.71% Oxygen
27.29% Carbon

Calculating Percent Composition

To simplify calculations, always assume that you are given 1 mole of a compound prior to calculations.

Step 1: Find molar mass of compound
Step 2: Divide molar mass of monoatomic element by compound's total molar mass.
Step 3: Multiply by 100%

Definition

The Empirical formula of a compound is listed in its simplest form of whole numbered ratios in a formula unit or atoms of each elements in a given molecule.

Example: The empirical formula for Ethene (C2H4) is written as: CH2

Calculating Empirical Formulas

Step 1: Convert the mass of each element in a compound to moles.
Step 2: Divide each of the mole values by the smallest number of moles that were calculated.
Step 3: Round to the nearest whole number.
Step 4: These are the subscripts for the given elements within the compound and represent the mole ratio.
Step 5: State formula.

Click On the Globe for some excellent examples

Calculating Empirical Formulas is also possible when given the Percent Composition of an element or compound. Utilize the same steps needed for both Empirical formulas and percent composition to solve these problems. Tip: Assume you have 100g of each substance for simplicity.

Definition

Molecular formulas is a multiple of a given empirical formula.
They are complicated and more detailed than empirical formulas, however, sometimes, they may be the same.

Calculating Molecular Formulas

The initial processes of calculation is similar to that of empirical formulas, however, final steps will differ.

Step 1: Calculate Empirical formula mass by the previously given methods.
Step 2: Divide the compound's molar mass by the empirical formula mass.
Step 3: Multiply every subscript in the empirical formula by the whole number found in step 2.
Step 4: State the newly found molecular formula.

Click on the Globe for some excellent examples

Definition

A hydrate is a substance or compound that contains water. Compounds that do not have water molecules are known as anhydrous.

Calculating Hydrates From Empirical Data

Step 1: Determine the mass of the water that has left the compound.
Step 2: Convert the mass of water into moles.
Step 3: Convert the mass of left over anhydrous to moles.
Step 4: Find the water to anhydrous ratio.
Step 5: Use the ratio to determine the new formula.

All of these steps require previous knowledge in empirical and molecular formulas (and even percent composition for some questions), so it is best to practice there, as you will need those skills to tackle hydrate problems.

Click on the Globe for a video example.