To go from atomic mass to moles, you first need to understand the relationship between atomic mass, molar mass, and moles. The atomic mass of an element, typically given in atomic mass units (amu), is numerically equal to its molar mass, which is expressed in grams per mole (g/mol). Let's break down the process:
Understanding Atomic Mass and Molar Mass
Atomic Mass
- Atomic mass is the mass of a single atom of an element, often expressed in atomic mass units (amu).
- It is found on the periodic table for each element.
Molar Mass
- Molar mass is the mass of one mole of a substance (element or compound) and is expressed in grams per mole (g/mol).
- According to the reference, the molar mass of an element is numerically equal to its atomic mass, but with units of g/mol instead of amu. For example, the atomic mass of carbon is about 12 amu; its molar mass is approximately 12 g/mol.
- Molar mass can be calculated by multiplying the atomic mass in amu by the molar mass constant (1 g/mol). This conversion gives the molar mass in g/mol.
- For compounds, molar mass is calculated by summing the atomic masses of all the constituent atoms.
Converting from Grams to Moles
Once you know the molar mass of an element or compound, you can calculate moles using the following:
Formula
Moles = Mass (g) / Molar Mass (g/mol)
Steps:
- Find the Atomic Mass: Locate the atomic mass of the element on the periodic table. This is numerically the same as the molar mass when converted to g/mol.
- Determine the Molar Mass: If you're working with a single element, the molar mass (g/mol) is the same number as the atomic mass (amu). For compounds, you sum up the molar masses of all the atoms in the compound.
- Example 1: Element – The atomic mass of Oxygen (O) is approximately 16 amu, so the molar mass of Oxygen is 16 g/mol.
- Example 2: Compound - To find the molar mass of water (H2O):
- Molar mass of Hydrogen (H) is about 1 g/mol (and there are two H's) so 2 * 1 g/mol = 2 g/mol
- Molar mass of Oxygen (O) is about 16 g/mol.
- Molar mass of H2O is approximately 2 g/mol + 16 g/mol = 18 g/mol
- Measure the Mass in Grams: You need the mass of the substance you're working with in grams.
- Calculate the Moles: Use the formula:
- Moles = Mass (g) / Molar Mass (g/mol)
Practical Example
Suppose you have 36 grams of water (H2O):
- We determined that the molar mass of water (H2O) is approximately 18 g/mol
- Moles = 36 g / 18 g/mol = 2 moles
Therefore, 36 grams of water is approximately 2 moles.
Key Takeaways
- Atomic mass, in amu, is numerically equal to molar mass, in g/mol for individual elements.
- Molar mass is essential for converting between mass in grams and the amount of a substance in moles.
- For compounds, sum up the atomic masses of each atom in its formula to calculate the compound's molar mass.
| Steps | Description |
|---|---|
| 1 | Get the atomic mass from the periodic table |
| 2 | Convert atomic mass (amu) to molar mass (g/mol) - same numeric value |
| 3 | Measure the mass of the substance in grams |
| 4 | Divide the mass by the molar mass to get moles |
