Volume fraction - the ratio of the volume of a solute to the volume of a solution. The volume fraction is measured in fractions of a unit or as a percentage.
where: V 1 - the volume of the dissolved substance, l;
V is the total volume of the solution, l.
As mentioned above, there are hydrometers designed to determine the concentration of solutions of certain substances. Such hydrometers are not graduated in terms of density, but directly in the concentration of the solution. For common solutions of ethyl alcohol, the concentration of which is usually expressed as a percentage by volume, such hydrometers are called alcohol meters or andrometers.
Molarity (molar volume concentration)
molarity concentration solution
Molar concentration - expressed in moles, the amount of a solute contained in one liter of a solution. Molar concentration in the SI system is measured in mol/m3, but in practice it is much more often expressed in mol/l or mmol/l.
Another designation for the molar concentration C M is possible, which is usually denoted M. Thus, a solution with a concentration of 0.5 mol / l is called 0.5 molar.
where: n is the amount of solute, mol;
V is the total volume of the solution, l.
The amount of substance in moles is the amount of substance equivalent to the number of moles of hydrogen ions or the number of moles of electrons in the corresponding reactions.
Molarity is calculated in two ways:
Method 1- according to the exact mass of a chemically pure substance using the formula:
M=a*1000/E*V,
where: a is the mass of a sample of a chemically pure substance, g;
E is the molar mass of the equivalent (conditional particles) of a chemically pure substance, g/mol;
V is the volume of the solution used for titration of the mass of the substance, ml;
1000 - the number of milliliters in 1 liter of solution.
Method 2- according to a titrated solution of known concentration using the formula:
M \u003d M 0 * V 0 / V,
where: M 0 is the molarity of the solution of the substance by which the titer is set (mol / l); V 0 - the volume of the solution, which sets the titer (ml); V is the volume of the solution, the molarity of which is set (ml).
Normal concentration (molar equivalent concentration)
Normal concentration - the number of equivalents of a given substance in 1 liter of solution. The normal concentration is expressed in mol-eq / l or g-eq / l (meaning mole equivalents). The abbreviations "n" or "N" are used to record the concentration of such solutions. For example, a solution containing 0.1 mol-eq / l is called decinormal and is written as 0.1 n.
where: n is the amount of solute, mol; V - the total volume of the solution, l; z is the equivalence number.
The normal concentration may differ depending on the reaction in which the substance is involved. For example, a one molar solution of H 2 SO 4 will be one normal if it is intended to react with an alkali to form KHSO 4 hydrosulfate, and two normal if it is to react to form K 2 SO 4 .
Volume fraction of gases in the mixture
1. Familiarize yourself, write down the definition and formula for finding the volume fraction of gas:
The composition of the air includes several different gases: oxygen, nitrogen, carbon dioxide, noble gases, water vapor and some other substances. The content of each of these gases in clean air is strictly defined.
In order to express the composition of a mixture of gases in numbers, that is, quantitatively, a special value is used, which is called the volume fraction of gases in the mixture.
The volume fraction of gas in the mixture is denoted by the Greek letter - "phi".
The volume fraction of a gas in a mixture is the ratio of the volume of a given gas to the total volume of the mixture:
What does the volume fraction of gas in the mixture show, or, as they say, what is the physical meaning of this quantity? The volume fraction of a gas shows what part of the total volume of the mixture is occupied by a given gas.
If we were able to separate 100 liters of air into separate gaseous components, we would get about 78 liters of nitrogen, 21 liters of oxygen, 30 ml of carbon dioxide, the remaining volume would contain the so-called noble gases (mainly argon) and some others (Fig. 62).
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Rice. 62. |
Let's calculate the volume fractions of these gases in the air:
It is easy to see that the sum of the volume fractions of all gases in the mixture is always equal to 1, or 100%:
(nitrogen) + (acid) + (carbon gas) + (other gases) = 78% + 21% + 0.03% + 0.97% = 100%.
The air that we exhale is much poorer in oxygen (its volume fraction decreases to 16%), but the carbon dioxide content increases to 4%. This air is no longer suitable for breathing. That is why a room in which there are many people must be regularly ventilated.
In chemistry in production, one often has to deal with the inverse problem: to determine the volume of gas in a mixture from a known volume fraction.
2. Review sample tasks
Example. Calculate the volume of oxygen contained in 500 liters of air.
From the definition of the volume fraction of gas in the mixture, we express the volume of oxygen:
V(sour) = V(air) (acid).
Substitute the numbers in the equation and calculate the volume of oxygen:
V(acid) \u003d 500 (l) 0.21 \u003d 105 l.
By the way, for approximate calculations, the volume fraction of oxygen in the air can be taken equal to 0.2, or 20%.
When calculating the volume fraction of gases in a mixture, you can use a little trick. Knowing that the sum of the volume fractions is 100%, for the "last" gas in the mixture, this value can be calculated differently.
Task. An analysis of the atmosphere of Venus showed that 50 ml of Venusian "air" contains 48.5 ml of carbon dioxide and 1.5 ml of nitrogen. Calculate the volume fractions of gases in the planet's atmosphere.
Given:
V(mixture) = 50 ml,
V(carbon gas) = 48.5 ml,
V(nitrogen) = 1.5 ml.
To find:
(coal gas),
Decision
Calculate the volume fraction of carbon dioxide in the mixture. A-priory:
Let us calculate the volume fraction of nitrogen in the mixture, knowing that the sum of the volume fractions of gases in the mixture is 100%:
(carbon gas) + (nitrogen) = 100%,
(nitrogen) = 100% - (carbon gas) = 100% - 97% = 3%.
Answer.(carbon gas) = 97%, (nitrogen) = 3%.
What quantity is used to measure the content of components in mixtures of another type, for example, in solutions? It is clear that in this case it is inconvenient to use the volume fraction. A new value comes to the rescue, which you will learn about in the next lesson.
3. Do your homework:
1. What is the volume fraction of a component in a gas mixture?
2. The volume fraction of argon in air is 0.9%. What volume of air is needed to produce 5 liters of argon?
3. When air was separated, 224 liters of nitrogen were obtained. What volumes of oxygen and carbon dioxide were obtained in this case?
4. The volume fraction of methane in natural gas is 92%. What volume of this gas mixture will contain 4.6 ml of methane?
5. Mixed 6 liters of oxygen and 2 liters of carbon dioxide. Find the volume fraction of each gas in the resulting mixture.
Lesson Objectives:
- To study the concept of mass and volume fraction of the components of a mixture and learn how to calculate them.
Lesson objectives:
Educational: to form an idea of the mass and volume fraction of the components of the mixture, to teach how to calculate these fractions;
Developing: to develop students' ability to analyze, solve problems, generalize, compare and draw conclusions;
Educational: expanding horizons.
Basic terms:
Mass fraction is the ratio of the mass of the solute to the total mass of the solution.
is the ratio of the volume of a given substance to the total volume of the mixture.
During the classes:
1. Among the given objects, choose the smallest in size:
b) a molecule;
c) poppy seed;
d) a grain of sand.
2. In what series do all the listed substances belong to simple ones?
a) chalk, carbon, ozone;
b) diamond, oxygen, granite;
c) sulfur, phosphorus, ozone;
3. A very important feature of the physical properties of water for wildlife is that:
a) the boiling point of water is 100º C;
b) the density of liquid water is higher than the density of ice;
c) the freezing point of water is 0º C;
d) water has a very low electrical conductivity.
4. Compounds containing only hydrogen and oxygen atoms:
a) none are known
b) only one is known;
c) several are known;
d) a huge amount is known.
5. When oxygen interacts with metals:
a) salts are formed;
b) ozone is released;
c) the resulting compounds are always oxides;
d) the resulting compounds are not always oxides.
solutions in nature.
The simplest solutions consist of two components. One of the components of the solution is a solvent. We are more familiar with liquid solutions, which means that the solvent in them is a liquid substance. Most often it is water.
You already know that natural water is never completely pure. So, there is water that contains a significant amount of calcium and magnesium salts and is called hard (there is also soft water, such as rainwater). Hard water gives little foam with soap, and scale forms on the walls of boilers and kettles when it is boiled. In Figure 1 you can see how hard water scales up. The hardness of water depends on the amount of salts dissolved in it. The content of a solute in a solution is expressed using its mass fraction.
Let's watch a video about water hardness:
The other component of the solution is the solute. It can be a gas, a liquid, or a solid.
In jewelry and technical products, not pure gold is used, but its alloys, most often with copper and silver. Pure gold - the metal is too soft, the nail leaves a mark on it. its wear resistance is low. The test, standing on gold products made in our country, means the mass fraction of gold in the alloy, more precisely, its content per thousand mass parts of the alloy. A sample of 583°, for example, means that the mass fraction of gold in the alloy is 0.583 or 58.3%.
Mass share.
One of the most common ways to express the concentration of a solution is in terms of the mass fraction of the solute.
The ratio of the mass of the solute to the total mass of the solution is called the mass fraction of the solute.
The mass fraction is denoted by the Greek letter "omega" and is expressed in fractions of a unit or percentage (Figure 2).
Fig.2. Mass fraction of the components of the mixture.
After watching the video
you will understand the concept of mass fraction and learn how to calculate it.
If 100 g of solution contains 30 g of sodium chloride, this means that ω(NaCl) = 0.3 or ω(NaCl) = 30%. You can also say: "There is a thirty percent solution of sodium chloride."
Mass fraction - the most common concentration in everyday life and most industries. It is the mass fraction of fat, for example, that is indicated on milk bags (look at Figure 3).
Fig.3. Mass fraction of fat in milk.
The mass of the solution is the sum of the mass of the solvent and the mass of the solute, i.e.:
m(solution) = m(solvent) + m(solute).
Suppose the mass fraction of the solute is 0.1, or 10%. Therefore, the remaining 0.9, or 90%, is the mass fraction of the solvent.
The mass fraction of a solute is widely used not only in chemistry, but also in medicine, biology, physics, and in everyday life. Consider the solution of some problems presented in Figures 4 and 5.
Fig.4. The task of finding the mass fraction.
Fig.5. The task of finding the mass fraction (in percent).
The composition of air includes several different gases: oxygen, nitrogen, carbon dioxide, noble gases, water vapor and some other substances. The content of each of these gases in clean air is strictly defined.
In order to express the composition of a mixture of gases in numbers, i.e. quantitatively, a special value is used, which is called the volume fraction of gases in the mixture.
Similarly, the mass fraction is determined by the volume fraction of a gaseous substance in a gas mixture, denoted by the Greek letter phi (Figure 6):
Rice. 6. Volume fraction.
The volume fraction of a gas shows what part of the total volume of the mixture is occupied by a given gas.
If we were able to separate 100 liters of air into separate gaseous components, we would get about 78 liters of nitrogen, 21 liters of oxygen, 30 ml of carbon dioxide, the remaining volume would contain the so-called noble gases (mainly argon) and some others (Figure 7 ).
Fig.7. Volume fraction of noble gases in air.
The air that we exhale is much poorer in oxygen (its volume fraction decreases to 16%), but the carbon dioxide content increases to 4%. This air is no longer suitable for breathing. That is why a room in which there are many people must be regularly ventilated.
In chemistry in production, one often has to deal with the inverse problem: to determine the volume of gas in a mixture from a known volume fraction.
Let's see how to solve problems for finding the volume fraction (Figure 8).
Fig.8. The problem of finding the volume fraction.
Findings.
1. The simplest solutions consist of two components. One of the components of the solution is a solvent. We are more familiar with liquid solutions, which means that the solvent in them is a liquid substance. The other component of the solution is the solute. It can be a gas, a liquid, or a solid.
2. One of the most common ways to express the concentration of a solution is through the mass fraction of a solute. The ratio of the mass of the solute to the total mass of the solution is called the mass fraction of the solute. The mass fraction is denoted by the Greek letter "omega" and is expressed in fractions of a unit or percentage.
3. The volume fraction of gas shows what part of the total volume of the mixture is occupied by this gas. The volume fraction of a gaseous substance in a gas mixture is denoted by the Greek letter phi.
control block.
1. What is the mass fraction of a solute?
2. What is the volume fraction of a component in a gas mixture?
3. Compare the concepts of "volume fraction" and "mass fraction" of the components of the mixture.
4. Mass fraction of iodine in pharmaceutical iodine tincture is 5%. What mass of iodine and alcohol should be taken to prepare 200 g of tincture?
5. The volume fraction of argon in air is 0.9%. What volume of air is needed to produce 5 liters of argon?
6. 25 g of table salt was dissolved in 150 g of water. Determine the mass fraction of salt in the resulting solution.
7. When air was separated, 224 liters of nitrogen were obtained. What volumes of oxygen and carbon dioxide were obtained in this case?
8. Mixed two solutions of sulfuric acid: 80 g of 40% and 160 g of 10%. Find the mass fraction of acid in the resulting solution.
Homework.
1. Make a report about pure matter and solutions in nature.
2. Give as many examples as possible of indicating the volume or mass fraction of a substance in a solution.
3. Come up with one problem each for finding the mass and volume fraction of a substance.
As you know, one of the saltiest bodies of water in the world is the Dead Sea. In it, the mass fraction of table salt NaCl can reach 10%, while in the Black Sea - no more than 1.8%. The molar concentrations of this salt are 3.3 mol/l and 0.5 mol/l, respectively. Thus, the mass fractions differ by about 5.5 times, and the molarity by 6.6 times. This is explained by the fact that the waters of the two seas have different densities: at the Dead Sea it is so great that it is almost impossible to drown in it; the density of the human body is less than the density of such a saline solution (Figure 9).
Fig.9. Dead Sea and swimming in it.
It is due to its high salt content that the Dead Sea is considered curative, as stated in this video:
Bibliography:
1. Lesson on the topic "Mass and volume fractions" Panina S.G., teacher of chemistry, secondary school No. 27, Arkhangelsk.
2. Lesson on the topic "Solution" Denisov A.N., chemistry teacher, gymnasium No. 3, Moscow.
3. Gabrielyan O.S. Chemistry. Grade 8: control and verification work for the textbook by O.S. Gabrielyan "Chemistry. 8" / O.S. Gabrielyan, P.N. Berezkin, A.A. Ushakova and others - M .: Bustard, 2006.
4. Gabrielyan O.S. Chemistry. Grade 8: textbook for educational institutions - M .: Bustard, 2008.
Edited and sent by Borisenko I.N.
Worked on the lesson:
Panina S.G.
Denisov A.N.
Borisenko I.N.
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Subjects > Chemistry > Chemistry Grade 8Concentration- value characterizing the quantitative composition of the solution.
The concentration of a solute (not a solution) is the ratio of the amount of a solute or its mass to the volume of a solution (mol / l, g / l), that is, this is the ratio of heterogeneous quantities.
Those quantities that are the ratio of the same type of quantities (the ratio of the mass of a solute to the mass of a solution, the ratio of the volume of a solute to the volume of a solution) are correctly called shares. However on practice for both types of expression of composition, the term is used concentration and talk about the concentration of solutions.
There are many ways to express the concentration of solutions.
Mass fraction (also called percentage concentration)
Mass fraction - the ratio of the mass of the solute to the mass of the solution. The mass fraction is measured in fractions of a unit.
m 1 - mass of the dissolved substance, g (kg);
m is the total mass of the solution, g (kg).
Mass fraction of the solute w (B) is usually expressed as a fraction of a unit or as a percentage. For example, the mass fraction of the dissolved substance - CaCl 2 in water is 0.06 or 6%. This means that a solution of calcium chloride weighing 100 g contains calcium chloride weighing 6 g and water weighing 94 g.
Example: How many grams of sodium sulfate and water are needed to prepare 300 g of a 5% solution?
Solution: m (Na 2 SO 4) \u003d w (Na 2 SO 4) / 100 \u003d (5 300) / 100 \u003d 15 (g)
where w (Na 2 SO 4)) is the mass fraction in%, m is the mass of the solution in g m (H 2 O) \u003d 300 g - 15 g \u003d 285 g.
Thus, to prepare 300 g of a 5% sodium sulfate solution, you need to take 15 g of Na 2 SO 4) and 285 g of water.
Mass percentage of the component, ω%
ω % =(m i /Σm i)*100
Volume fraction
Volume fraction - the ratio of the volume of a solute to the volume of a solution. The volume fraction is measured in fractions of a unit or as a percentage.
V 1 - the volume of the dissolved substance, l;
V is the total volume of the solution, l.
There are hydrometers designed to determine the concentration of solutions of certain substances. Such hydrometers are graduated not in density values, but directly in the values of the concentration of the solution. For common solutions ethyl alcohol, the concentration of which is usually expressed as a percentage by volume, such hydrometers are called alcohol meters.
Molarity (molar volume concentration)
Molar concentration - the amount of solute (number of moles) per unit volume of the solution. Molar concentration in is measured in mol/l (M) or mmol/l (mM). The expression in "molarity" is also common. So, a solution with a concentration of 0.5 mol / l is called 0.5 molar.
ν - amount of dissolved substance, mol;
V is the total volume of the solution, l.
Molar concentration is measured in mol/l and is denoted by "M". For example, 2 M NaOH is a 2 molar solution of sodium hydroxide. One liter of such a solution contains 2 mol of a substance or 80 g.
Example: What mass of potassium chromate K 2 CrO 4 need to take to prepare 1.2 liters of 0.1 M solution?
Solution: M (K 2 CrO 4) \u003d C (K 2 CrO 4) V M (K 2 CrO 4) \u003d 0.1 mol / l 1.2 l 194 g / mol "23.3 g.
Thus, to prepare 1.2 liters of a 0.1 M solution, you need to take 23.3 g of K 2 CrO 4 and dissolve in water, and bring the volume to 1.2 liters.
