Most substances we encounter are mixtures - wood, gas, milk, champagne, air, steel, etc...
When the components of a mixture are uniformly intermingled or mixed, the homogeneous mixture is a solution.
Solutions can be dilute or concentrated, but we need to define "solution composition" more precisely to do calculations.
There are several different ways to define a solution's concentration.
Types of Concentration
Here are five types of concentration...
- Mass Percent
- Mole Fraction
Let's take a closer look at each of these 5 types of concentration, and their respective formulas.
The molarity of a solution is equal to the moles of solute per one liter of solution.
Visually, we have the molarity formula below:
Molarity is the most common way of determining (and providing) a solution's "composition" or concentration.
The molality of a solution is equal to the moles of solute per one kilogram of solvent.
Here's how the molality formula looks:
Most solutions in General Chemistry are dissolved in water, which means the solvent is H2O.
So molality often represents the amount of solute (in moles) per kilogram of water.
Mass Percent Formula
The mass percent formula of a solution is equal to the mass of the solute divided by the mass of the entire solution (both solute + solvent), times 100%.
Here's the formula for mass percent:
This is just one chemistry teacher's opinion but, after molarity, mass percent is the second most common way to express the concentration of a solution in General Chemistry.
Mole Fraction Formula
The mole fraction of a solution is equal to the moles of solute divided by the moles of the entire solution (moles of solute + moles of solvent).
Visually, the mole fraction formula looks like this:
Mole fraction is represented by an upper-case Greek letter "chi," which looks like a "wobbly X."
The normality of a solution is equal to the number of equivalents of solute per one liter of solution.
The normality formula is here:
Quite often, but not always, the normality of a solution is reserved for strong acid solutions and/or strong base solutions.
Mass Percent Calculations
Molarity calculation examples and mole fraction calculations can be found in my other videos.
But today, let's examine a mass percent calculation, followed by a typical molality calculation.
Here we go...
ex: A sulfuric acid solution is 3.75 M and has a density of 1.230 g/mL.
Calculate the mass percent and molality of the sulfuric acid.
➞ To find mass percent we need the mass of solute (H2SO4) and the mass of the solution:
molarity = 3.75 moles H2SO4 / 1 L solution,
(1.230 g solution / 1 mL solution) x (1000 mL / 1 L)
= 1230. g/L
➞ So, 1 L of solution contains 1230. g of solution.
➞ Hold that for a second.
➞ Now, let's find the mass of solute, H2SO4, followed by the mass percent of H2SO4 .
(3.75 mol H2SO4 / 1) x (98.1 g H2SO4 / 1 mol H2SO4)
= 368 g H2SO4 (solute)
mass % = [ (368 g H2SO4) / (1230. g solution) ] x 100%
= 29.9 % H2SO4
You can see it clearly below:
REMEMBER, we still have to do the second part of the problem: calculating the molality...
➞ To calculate molality, we need moles of solute (which we have) and kg solvent.
1230. g solution - 368 g solute = 862 g H2O solvent
This is equal to = 0.862 kg H2O
➞ Molality = (moles H2SO4) / (1 kg H2O)
= 3.75 moles / 0.862 kg
= 4.35 m H2SO4
Here's the above molality calculation, shown more visually:
Stick around for my next post from SECTION 11 - Solutions and Their Properties.
We'll discuss the Three Factors that Affect the Solubility of a Solution.