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Types of Concentration. How to Calculate Molality and Mass Percent

SECTION 11 - Solutions and Their Properties


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 Formula

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:

Molality Formula

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:

Mass Percent Formula

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 Formula

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:

Normality Formula

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.

Mass Percent Calculations

➞ 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:

Molarity and Mass Percent Examples

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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:

How to Calculate Molality with Examples

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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.