Molar and molar concentrations, despite similar names, are different. Their main difference is that when determining the molar concentration, the calculation is carried out not on the volume of the solution, as when detecting molarity, but on the mass of the solvent.
General Information on Solutions and Solubility
A true solution is a homogeneous system that includes a number of components that are independent of each other. One of them is considered a solvent, and the rest are substances dissolved in it. The solvent is the substance that is most in the solution.
Solubility - the ability of a substance to form homogeneous systems with other substances - solutions in which it is in the form of individual atoms, ions, molecules or particles. And concentration is a measure of solubility.
Therefore, solubility is the ability of substances to be distributed evenly in the form of elementary particles throughout the volume of the solvent.
True solutions are classified as follows:
- by type of solvent - non-aqueous and aqueous;
- by type of solute - solutions of gases, acids, alkalis, salts, etc .;
- by interaction with electric current - electrolytes (substances that have electrical conductivity) and non-electrolytes (substances not capable of electrical conductivity);
- by concentration - diluted and concentrated.
Concentration and methods of expression
Concentration refers to the content (weight) of a substance dissolved in a certain amount (weight or volume) of a solvent or in a certain volume of the whole solution. It is of the following types:
1. The percentage concentration (expressed in%) - it indicates how many grams of solute are contained in 100 grams of solution.
2. The molar concentration is the number of gram moles per 1 liter of solution. Shows how many gram molecules are contained in 1 liter of a substance solution.
3. Normal concentration is the number of gram equivalents per 1 liter of solution. Shows how many gram equivalents of solute are contained in 1 liter of solution.
4. The concentration of the molar indicates how much solute in moles per 1 kilogram of solvent.
5. The titer determines the content (in grams) of the substance, which is dissolved in 1 milliliter of solution.
Molar and molar concentrations differ from each other. Consider their individual characteristics.
Molar concentration
The formula for its definition:
Cv = (v / V), where
v is the amount of solute, mol;
V is the total volume of the solution, liter or m 3 .
For example, the entry “0.1 M solution of H 2 SO 4” indicates that 0.1 mol (9.8 grams) of sulfuric acid is present in 1 liter of such a solution .
Molar concentration
You should always take into account the fact that the molar and molar concentrations have completely different meanings.
What is the molar concentration of the solution? The formula for its definition is as follows:
Cm = (v / m), where
v is the amount of solute, mol;
m is the mass of solvent, kg
For example, a record of 0.2 M NaOH solution means that 0.2 moles of NaOH are dissolved in 1 kilogram of water (in this case, it is a solvent).
Additional formulas needed for calculations
A lot of supporting information may be required in order to calculate the molal concentration. Formulas that may be useful for solving basic problems are presented below.
By the quantity of substance ν is understood a certain number of atoms, electrons, molecules, ions or its other particles.
v = m / M = N / N A = V / V m , where:
- m is the mass of the compound, g or kg;
- M is the molar mass, g (or kg) / mol;
- N is the number of structural units;
- N a - the number of structural units in 1 mole of substance, Avogadro constant: 6.02 . 10 23 mol - 1 ;
- V is the total volume, l or m 3 ;
- V m - molar volume, l / mol or m 3 / mol.
The latter is calculated by the formula:
V m = RT / P, where
- R is a constant, 8.314 J / (mol . K);
- T is the gas temperature, K;
- P is the gas pressure, Pa.
Examples of tasks on molarity and molality. Task number 1
Determine the molar concentration of potassium hydroxide in a solution of 500 ml. The mass of KOH in the solution is 20 grams.
Definition
The molar mass of potassium hydroxide is:
M KOH = 39 + 16 + 1 = 56 g / mol.
We calculate how much potassium hydroxide is contained in the solution:
ν (KOH) = m / M = 20/56 = 0.36 mol.
We take into account that the volume of the solution should be expressed in liters:
500 ml = 500/1000 = 0.5 liters.
Determine the molar concentration of potassium hydroxide:
Cv (KOH) = v (KOH) / V (KOH) = 0.36 / 0.5 = 0.72 mol / liter.
Task number 2
How much sulfur (IV) oxide under normal conditions (i.e. when P = 101325 Pa, and T = 273 K) do I need to take in order to prepare a solution of sulfur dioxide with a concentration of 2.5 mol / liter with a volume of 5 liters?
Definition
Determine how much sulfuric acid is contained in the solution:
ν (H 2 SO 3 ) = Cv (H 2 SO 3 ) ∙ V (solution) = 2.5 ∙ 5 = 12.5 mol.
The equation for producing sulfurous acid has the following form:
SO 2 + H 2 O = H 2 SO 3
According to this:
ν (SO 2 ) = ν (H 2 SO 3 );
ν (SO 2 ) = 12.5 mol.
Bearing in mind that under normal conditions 1 mol of gas has a volume of 22.4 liters, we calculate the volume of sulfur oxide:
V (SO 2 ) = ν (SO 2 ) ∙ 22.4 = 12.5 ∙ 22.4 = 280 liters.
Task number 3
Determine the molar concentration of NaOH in the solution at its mass fraction equal to 25.5% and a density of 1.25 g / ml.
Definition
We take as a sample a solution with a volume of 1 liter and determine its mass:
m (solution) = V (solution) ∙ p (solution) = 1000 ∙ 1.25 = 1250 grams.
We calculate how much alkali in the sample by mass:
m (NaOH) = (w ∙ m (solution)) / 100% = (25.5 ∙ 1250) / 100 = 319 grams.
The molar mass of sodium hydroxide is equal to:
M NaOH = 23 + 16 + 1 = 40 g / mol.
We calculate how much sodium hydroxide is contained in the sample:
v (NaOH) = m / M = 319/40 = 8 mol.
Determine the molar concentration of alkali:
Cv (NaOH) = v / V = 8/1 = 8 mol / liter.
Task number 4
10 grams of NaCl salt was dissolved in water (100 grams). Set the concentration of the solution (molal).
Definition
The molar mass of NaCl is equal to:
M NaCl = 23 + 35 = 58 g / mol.
The amount of NaCl contained in the solution:
ν (NaCl) = m / M = 10/58 = 0.17 mol.
In this case, the solvent is water:
100 grams of water = 100/1000 = 0.1 kg of H 2 O in this solution.
The molar concentration of the solution will be equal to:
Cm (NaCl) = v (NaCl) / m (water) = 0.17 / 0.1 = 1.7 mol / kg.
Task number 5
Determine the molar concentration of a 15% solution of alkali NaOH.
Definition
A 15% alkali solution means that every 100 grams of solution contains 15 grams of NaOH and 85 grams of water. Or that in every 100 kilograms of the solution there is 15 kilograms of NaOH and 85 kilograms of water. In order to cook it, it is necessary to dissolve 15 grams (kilograms) of alkali in 85 grams (kilograms) of N 2 O.
The molar mass of sodium hydroxide is equal to:
M NaOH = 23 + 16 + 1 = 40 g / mol.
Now we find the amount of sodium hydroxide in the solution:
ν = m / M = 15/40 = 0.375 mol.
Solvent (water) mass in kilograms:
85 grams of H 2 O = 85/1000 = 0.085 kg of H 2 O in this solution.
After this, the molal concentration is determined:
Cm = (ν / m) = 0.375 / 0.085 = 4.41 mol / kg.
In accordance with these typical tasks, it is possible to solve the majority of others to determine molality and molarity.