Concentration
The term "concentration" describes how much solute is contained in a specific volume of solvent or solution. It measures a substance's relative density or abundance within the mixture.
Making precise volume solutions, usually referred to as percent solutions, is the main application for a volumetric flask. It is specially made to deliver an accurate volume measurement with a small margin of error, often at a set volume. On the other hand, a volumetric flask offers more precision than a measuring cylinder, despite the latter's usage for volume measurements.
Units of Concentration
There are several units commonly used to measure
concentration, depending on the nature of the mixture and the properties of the
substances involved. Here are some commonly used units of concentration:
1.
Molarity (M):
Molarity measures the number of moles of solute present or dissolved in one liter of solution. It is expressed as moles per liter (mol/L or M). For example, a solution with a molarity of 0.22 M means that there are 0.222 moles of solute dissolved in every liter of solution.
2.
Mass Percent (%):
Mass percent represents the percentage of the mass of
the solute in a solution. It is calculated by dividing the mass of the solute
by the mass of the solution and multiplying by 100%. Mass percent is denoted by
the symbol "%". For example, a mass percent of 20% means that 20 grams of solute are present in 100 grams of solution.
3.
Molality (m):
Molality measures the concentration of a solute in
terms of its moles per kilogram of solvent. It is expressed as moles per
kilogram (mol/kg). Molality is particularly useful when the volume of the
solution changes with temperature, as it is not affected by changes in volume.
4.
Normality (N):
Normality is a measure of concentration that considers
the equivalent weights of solute species involved in a chemical reaction. It is
expressed in equivalents per liter (eq/L or N).
5. Parts Per Million (ppm) or Parts Per
Billion (ppb):
These units are used when dealing with extremely dilute solutions. PPM and PPB indicate the number of parts of the solute in one million or one billion parts of the solution, respectively.
These are just a few examples of the units used to
measure concentration, and there may be specific units used for different
properties or industries. It is important to use the appropriate unit for the
specific context or calculation at hand.
Mass
Percent (%) Solution
To make percent solutions, we need to understand the
different ways in which the percentage is calculated: m/m (mass/mass), m/v
(mass/volume), v/m (volume/mass), and v/v (volume/volume).
1.
m/m (mass/mass):
This means the mass of the solute is expressed as a
percentage of the total mass of the solution.
To make a m/m percent solution, you need to
weigh the solute (in grams) and the total solution (including the solute) to
calculate the mass percentage.
% m/m = (mass of solute / mass of solution) x 100
Point
To Remember
In a mass-by-mass percent solution, we must weigh both the solute and the solution. A weight balance must be used in order to precisely calculate the solution's mass. The entire mass of the solution may not be precisely measured with a measuring flask alone. Instead of measuring masses, a measuring flask is often used to measure volumes.
We can precisely determine the % concentration by using a weight balance to measure the mass of the solution in question. This makes sure that the right ratios of solute and solvent are employed to get the concentration you want. Therefore, while producing a mass-by-mass percent solution, it is advised to use a weight balance to determine the mass of the solution.
2. m/v (mass/volume):
This means the mass of the solute is expressed as a
percentage of the total volume of the solution.
- To make a m/v percent solution, you need to
weigh the solute (in grams) and measure the volume of the solution (in
milliliters) to calculate the mass/volume percentage.
% m/v = (mass of solute / volume of solution) x 100
3.
v/m (volume/mass):
This means the volume of the solute is expressed as a
percentage of the total mass of the solution.
To make a v/m percent solution, we first measure the volume of the solute (in milliliters) and weigh the total solution
(including the solute) to calculate the volume/mass percentage.
% v/m = (volume of solute / mass
of solution) x 100
It is necessary to use a weight balance for m/m
(mass/mass) and v/m (volume/mass) percent solutions. In both cases, we need to
measure the mass of the solute, and for v/m solutions, we also need to measure
the mass of the solution.
Using a weight balance ensures accurate measurements of the
mass, which is crucial for calculating the percentage accurately. It is
recommended to use a precision balance that can measure small masses with high
accuracy for precise results.
However, for m/v (mass/volume) and v/v (volume/volume)
percent solutions, we don't necessarily need a weight balance. For m/v
solutions, we only need to measure the mass of the solute and the volume of
the solution. For v/v solutions, we need to note or measure the volume of both the
solute and the solution. In these cases, we can use a graduated cylinder or a
volumetric flask with appropriate markings to measure the volume accurately.
4.
v/v (volume/volume):
This means the volume of the solute is expressed
as a percentage of the total volume of the solution.
To prepare a v/v percent solution, we first need to
measure the volume of the solute (in milliliters) and the volume of the
solution (in milliliters) to calculate the volume/volume percentage.
% v/v = (volume of solute / volume of
solution) x 100
In each case, after calculating the percentage, one can add the solute to the solvent and mix thoroughly to create the percent
solution.
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