To find the number of moles of a substance, you need to know its mass and its molecular weight. You then divide the mass by the molecular weight. This is explained in the reference material.
Understanding Moles
Before diving into the calculation, let's briefly define what a mole is. A mole is a unit of measurement in chemistry used to express amounts of a chemical substance. One mole contains exactly 6.02214076 × 1023 elementary entities (Avogadro's number). These entities can be atoms, molecules, ions, or other particles.
Calculating Moles
The core principle for finding moles is quite simple. Here’s a detailed breakdown:
The Formula
The number of moles (n) can be calculated using the following formula:
n = m / M
Where:
- n = number of moles
- m = mass of the substance (usually in grams)
- M = molar mass of the substance (grams per mole)
Step-by-Step Guide
- Measure the mass: Use a balance to accurately measure the mass (m) of your substance in grams.
- Determine the molecular weight (Molar Mass):
- Find the molar mass of the substance, which is the mass of one mole of that substance. This is often done by adding the atomic weights of all the atoms in the molecular formula, which you can obtain from the periodic table. For example, the molar mass of water (H₂O) is approximately 18 g/mol. (2 x H = 2 x 1.008 g/mol, 1 x O = 1 x 16.00 g/mol. 2.016 + 16.00 = 18.016 g/mol).
- Divide mass by molecular weight: Divide the measured mass (m) of the substance by its molar mass (M).
Example
Let's say you have 36 grams of water (H₂O). You want to determine how many moles of water you have.
- Mass (m): 36 g
- Molar Mass (M): 18 g/mol (approximately, calculated above)
n = m / M
n = 36 g / 18 g/mol
n = 2 moles
Therefore, there are 2 moles of water in 36 grams of water.
Practical Insights
- Always ensure you're using the correct units (grams for mass and grams per mole for molar mass).
- Molar mass is substance-specific; it's essential to find the correct molar mass for your compound.
- You may need to use the periodic table to determine the molar masses of individual elements and their relative amounts in a compound.
Summary
| Step | Description |
|---|---|
| 1 | Measure the mass (m) of the substance in grams. |
| 2 | Determine the molar mass (M) of the substance in grams per mole. |
| 3 | Divide the mass (m) by the molar mass (M): n = m / M, where n is the number of moles |
