ASP - Moles, Mass, and Particles (Lesson)
Moles, Mass, and Particles
Counting Particles of Matter
To get started, you need to secure your understanding of moles, mass, and particle calculations.
The mole is a counting unit. There are many "counting" units. Terms such as a dozen, a ream (of paper), or a gross, which is a dozen dozens or 144 items, are all types of counting units that may be more familiar than moles.
One mole is an enormous number of items - 602 sextillion items! This huge count makes dozens, pairs, and reams look trivial!
The mole (abbreviated mol) is the SI measure of the quantity of a pure substance such as atoms, molecules, or ions. It is defined as the amount of a substance that contains as many particles as there are atoms in 12 grams of pure carbon-12. So, 1 mole contains 6.022×1023 elementary entities of the substance, with the entities being atoms for elements, molecules for molecular compounds, formula units for salts, and ions for charged atoms.
The mass of one dozen eggs is about 680 grams. (The ones marked "large" in the store.) How many grams is one mole of something? Well, we cannot work with eggs, since they are not pure substances, but there is water in eggs, and water IS a pure substance, so let's consider water... How many grams is a mole of water? It is 18 grams!
Where Does That Number Come From?
When we put the idea of the counting unit Mole with the amount of matter Mass, we get a special property of pure substances. If the substance is an element, this might be called atomic mass, if the pure substance is a salt, it might be called formula unit mass. Molecular compounds like water would have an 18-gram quantity called molecular mass. Too many terms and not really all necessary (but you may see them in resources). We can simplify the situation by using the term: MOLAR MASS in place of all of them.
Molar Mass: The mass, in grams, of 1 MOLE of any pure substance. The Mole is a metric counting unit. So, the molar mass is the amount of matter stated in grams, of one mole of some element or compound.
Please watch the video below to learn more about molar mass. Be sure your volume is turned on!
Calculating Molar Mass
Sucrose has the structural formula shown that simplifies to the chemical formula C12H22O11. To find the molecular mass of this compound the chemical formula shows that in one mole of sucrose, there are 12 moles of carbon atoms, 22 moles of hydrogen atoms, and 11 moles of oxygen atoms.
Steps for Solving for Molar Mass
- Count the atoms in the chemical formula.
- Find the atomic mass of each on the periodic table.
- Multiply the atom count by the atomic mass for each.
- Find the sum of products.
Let's find the molar mass of sucrose:
When the chemical formula requires parentheses to indicate multiple polyatomic ions in one formula unit of a salt, be sure to use the subscript following the parentheses to find the total count. Let's look at barium hydroxide for example.
Conversion Factors
A conversion factor is a ratio (fraction) that represents the relationship between the same quantity in two different units.
There are two possible conversion factors for every ratio: both the ratio and its inverse.
Conversion factors are used to convert an amount from one unit to a different unit. For example, molar mass is the conversion factor used to convert from moles to grams and Avogadro's number is part of a conversion factor used to convert between moles and the number of particles.
Using our sucrose example from the earlier example, let's write two conversion factors from the information we found by changing the equality found for the mass on one mole into a fraction.
How To Use Conversion Factors
When selecting a conversion factor and considering which two ratios to use, take note of the given quantity. The value that corresponds to the given quantity is the denominator in conversion. The top of the ratio will have the unit needed as the numerator. For example:
How To Report Answers
The significant digits in the answer will be limited by the least precise value found in the entire calculation. That should always be the given values. Strive to use enough digits in your conversion factors so that Avogadro's number or molar mass does not limit the calculated value in the answer.
NOTE: The quantity of 1 mole is not a measurement and does not limit precision. Saying "1 mole equals some value" means EXACTLY 1, with infinite precision.
You Try It!
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