Chemistry
Unit 6: Composition of Substances and Solutions
See also Chemistry: Atoms First 2e, Chapter 6
A substance is a material that has a set ratio of each type of atom in specific arrangments, and a solution is a mixture that allows for different ratios of the same atoms or molecules. In this unit, we look at how to calculate and describe these ratios.
Moles
Before we delve into the compositions of materials, you need to first understand the concept of the mole. A mole gives us a quantity that helps simplify our calculations.
A mole is set so that if you have approximately one mole of protons or one mole of neutrons, then you'll get 1 gram of material. So if you have 1 mole of helium — which has two protons and two neutrons — then you'll have four grams of helium.
So how many atoms of helium does this give you? 6.02 x 1023 atoms. That is a lot of atoms — but still only a gram. Yet that's enough to fill 5.6 liters inside a balloon.
Steve Mould | "An Actually Good Explanation of Moles"
Formula Mass
The formula mass is the measure of the mass of atoms needed to define a substance. Other terms for this are molar mass, molecular weight, molecular weight, and formula weight. Formula mass is technically the most accurate description because weight can change depending upon gravity, and not all substances form discrete molecules. However, the other terms are very common, so they are often used interchangably (as pointed out in the videos.)
Formala mass measures the total average mass from all the atoms in the compound. For a compound, you need the smallest unit of the material:
In covalent compounds (like water), formula mass is the mass of a single molecule.
In ionic compounds (like salts), you find the lowest common denominator for each atom, and then use that to determine the formula mass. Since ionic compounds form crystal lattices rather than molecules, you don't call this the molecular mass.
Don't forget that elements have multiple isotopes. When you add up the masses of the atoms, you're actually adding together the average of the naturally occurring isotopes.
Wayne Breslin | "How to Calculate Molar Mass (Molecular Weight)"
Tyler DeWhitt | "How to Calculate Molar Mass Practice Problems"
Deriving Formulas from Empirical Mass
The empirical mass provides the ratio of elements in a molecule. So if you have a substance, and you know the relative masses of each element, you can use those masses to determine the ratios of each atom.
This diagram from Chemistry 2e: Atoms First gives the process for determining and empirical formula using only the masses of atoms in a given substance:
The empirical formula only gives the ratios of elements — it doesn't give the absolute forumla. To find the absolute formula for a covalent molecule, you also need to know the mass of each molecule. Using that mass, you can determine just how many times the empirical formula will fit into the total molecular mass.
Calculating Step-By-Step
In Melissa Maribel's video, she describes this process step-by-step, showing the necessary calculations.
Melissa Maribel | "Calculating Molecular Formulas Step by Step"
Molarity and Solution Concentrations
A solution is a mixture of different substances in a liquid. Unlike substances, a solution can have different ratios of atoms. So when we describe a solution, it's not like a molecule — we can't just say what atoms are in the mixture. Instead, we have to give the quantities that have been mixed.
Solvent: This is the liquid you're using to dissolve things. So if you add some instant coffee and sugar to a mug of hot water, the water is the solvent.
Solutes are the substances you add to the solvent. So your instant coffee and sugar would be solutes.
A solution is a homogenous mixture. This means that when you add solutes to solvents, the solutes must completely dissolve to be included in the solution.
The concentration of the solution is a measure of how much solute you've dissolved in the solvent. We measure this with Molarity (M):
molarity is a measure of how many moles of solute you have per liter of solvent
In the video from Crash Chemistry, we see how changing the concentrations affect Molarity, as well as how to do the calculations.
Crash Chemistry | "Concentration and Molarity"