S11E1 - Types of Concentration. How to Calculate Molality and Mass Percent

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.

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Solution Concentration

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.

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Types of Concentration

Here are five types of concentration...

• Molarity
• Molality
• Mass Percent
• Mole Fraction
• Normality

Let's take a closer look at each of these 5 types of concentration, and their respective formulas.

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Molarity Formula

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.

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Molality Formula

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.

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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.

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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."

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Normality Formula

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.

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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.
_________
answer:

➞ 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,

and...

(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 .  

We have:

(3.75 mol H₂SO₄ / 1) x (98.1 g H₂SO₄ / 1 mol H₂SO₄)

=  368 g H₂SO₄ (solute)

➞ Finally,

mass % =  [ (368 g H₂SO₄) / (1230. g solution) ] x 100%

=  29.9 % H₂SO₄ 

You can see it clearly below:

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REMEMBER, we still have to do the second part of the problem: calculating the molality...

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Molality Calculations

➞ 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:

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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.