Education, Secondary education and schools
Mole concentration. What does molar and molar concentration mean?
Molar and molal concentrations, despite the similar names, the values are different. Their main difference is that when calculating the molar concentration, the calculation is made not on the volume of the solution, as in the case of molarity, but on the mass of the solvent.
General information about solutions and solubility
A true solution is a homogeneous system that includes a number of components independent of each other. One of them is considered a solvent, and the rest are substances dissolved in it. The solvent is considered to be the substance that is most in solution.
Solubility - the ability of a substance to form with other substances homogeneous systems - solutions in which it is in the form of individual atoms, ions, molecules or particles. And concentration is a measure of solubility.
Consequently, solubility is the ability of substances to be distributed uniformly in the form of elementary particles throughout the volume of the solvent.
The true solutions are classified as follows:
- By type of solvent - non-aqueous and aqueous;
- By the form of the dissolved substance - solutions of gases, acids, alkalis, salts, etc .;
- On the interaction with the electric current - electrolytes (substances that have electrical conductivity) and non-electrolytes (substances not capable of electrical conductivity);
- By concentration - diluted and concentrated.
Concentration and ways of expressing it
1. Concentration percentage (expressed in%) - it indicates how much grams of solute in 100 grams of solution are contained.
2. Molar concentration is the number of gram-moles per 1 liter of solution. Shows how much gram molecules are contained in 1 liter of the solution of the substance.
3. Concentration is the number of gram-equivalents per 1 liter of solution. It shows how much gram-equivalents of solute are contained in 1 liter of solution.
4. Molar concentration shows how much solute in moles is per 1 kilogram of solvent.
5. The titer determines the content (in grams) of the substance, which is dissolved in 1 milliliter of the solution.
The molar and molar concentrations differ from each other. Let us consider their individual characteristics.
The molar concentration
The formula for its definition:
Cv = (v / V), where
V - amount of dissolved substance, mole;
V is the total volume of the solution, liter or m 3 .
For example, the record "0.1 M solution of H 2 SO 4" indicates that in 1 liter of such a solution there is 0.1 mol (9.8 grams) of sulfuric acid .
Mole concentration
It should always be taken into account 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 - amount of dissolved substance, mole;
M is the mass of the solvent, kg.
For example, recording a 0.2 M solution of NaOH means that in one kilogram of water (in this case it is a solvent) 0.2 M NaOH is dissolved.
Additional formulas required for calculations
A lot of auxiliary information may be required in order to calculate the molar concentration. Formulas that can be useful for solving basic problems are presented below.
The quantity of matter ν is understood to mean a certain number of atoms, electrons, molecules, ions or other particles.
V = m / M = N / N A = V / V m , where:
- M is the mass of the compound, g or kg;
- M - molar mass, g (or kg) / mol;
- N is the number of structural units;
- N A - number of structural units in 1 mole of substance, constant Avogadro: 6.02 . 10 23 mol - 1 ;
- V is the total volume, l or m 3 ;
- V m - volume molar, 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 temperature of the gas, K;
- P is the gas pressure, Pa.
Examples of problems of molarity and molality. Task number 1
Determine the molar concentration of potassium hydroxide in a 500 ml solution. The KOH in solution is 20 grams.
Definition
The molar mass of potassium hydroxide is:
М КОН = 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 oxide (IV) under normal conditions (ie when P = 101325 Pa, and T = 273 K) should be taken in order to prepare a solution of sulfurous acid with a concentration of 2.5 moles / liter 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 the production of sulfuric acid is as follows:
SO 2 + H 2 O = H 2 SO 3
According to this:
Ν (SO 2 ) = ν (H 2 SO 3 );
Ν (SO2) = 12.5 mol.
Bearing in mind that under normal conditions, 1 mole of gas has a volume of 22.4 liters, we calculate the volume of sulfur oxide:
V (SO2) = ν (SO2) ∙ 22.4 = 12.5 ∙ 22.4 = 280 liters.
Task number 3
Determine the molar concentration of NaOH in the solution at a mass fraction of 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 in the sample alkali by weight:
M (NaOH) = (w ∙ m (solution)) / 100% = (25.5 ∙ 1250) / 100 = 319 grams.
The molar mass of sodium hydroxide is:
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
In water (100 grams), 10 grams of NaCl salt was dissolved. Set the concentration of the solution (molal).
Definition
The molar mass of NaCl is:
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:
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% solution of alkali means that every 100 grams of the 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 make it, you need to dissolve 15 grams (kilogram) of alkali in 85 grams (kilograms) of H 2 O.
The molar mass of sodium hydroxide is:
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.
Mass of solvent (water) in kilograms:
85 grams H 2 O = 85/1000 = 0.085 kg H 2 O in this solution.
After this, the molar concentration is determined:
Cm = (ν / m) = 0.375 / 0.085 = 4.41 mol / kg.
In accordance with these typical problems, most others can be solved for the determination of molality and molarity.
Similar articles
Trending Now