Molar mass of chemical elements table. Periodic system of Mendeleev
The masses of atoms and molecules are very small, so it is convenient to choose the mass of one of the atoms as a unit of measurement and express the masses of the remaining atoms relative to it. This is exactly what the founder of the atomic theory Dalton did, who compiled a table of atomic masses, taking the mass of a hydrogen atom as a unit.
Until 1961, in physics, 1/16 of the mass of an oxygen atom 16 O was taken as an atomic mass unit (abbreviated amu), and in chemistry - 1/16 of the average atomic mass of natural oxygen, which is a mixture of three isotopes. The chemical mass unit was 0.03% larger than the physical one.
Atomic mass and relative atomic mass of an element
At present, a unified measurement system has been adopted in physics and chemistry. 1/12 of the mass of the carbon atom 12 C is chosen as the standard unit of atomic mass.
1 amu \u003d 1/12 m (12 C) \u003d 1.66057 × 10 -27 kg \u003d 1.66057 × 10 -24 g.
DEFINITION
Relative atomic mass of an element (A r) is a dimensionless quantity equal to the ratio medium weight an atom of an element to 1/12 of the mass of an atom 12 C.
When calculating the relative atomic mass, the abundance of isotopes of elements in earth's crust. For example, chlorine has two isotopes 35 Cl (75.5%) and 37 Cl (24.5%). The relative atomic mass of chlorine is:
A r (Cl) \u003d (0.755 × m (35 Cl) + 0.245 × m (37 Cl)) / (1/12 × m (12 C) = 35.5.
From the definition of relative atomic mass it follows that the average absolute mass of an atom is equal to the relative atomic mass times the amu:
m(Cl) = 35.5 × 1.66057 × 10 -24 = 5.89 × 10 -23 g.
Relative molecular weight of an element
DEFINITION
Relative molecular weight of the substance (M r)- this is a dimensionless quantity equal to the ratio of the mass of a molecule of a substance to 1/12 of the mass of an atom 12 C.
The relative molecular mass of a molecule is equal to the sum of the relative atomic masses of the atoms that make up the molecule, for example:
M r (N 2 O) = 2×A r (N) + A r (O) = 2×14.0067 + 15.9994 = 44.0128.
The absolute mass of a molecule is equal to the relative molecular mass times the amu.
The number of atoms and molecules in ordinary samples of substances is very large, therefore, when characterizing the amount of a substance, a special unit of measurement is used - the mole.
A mole is the amount of a substance that contains as many particles (molecules, atoms, ions, electrons) as there are carbon atoms in 12 g of the 12 C isotope.
The mass of one 12 C atom is 12 amu, so the number of atoms in 12 g of the 12 C isotope is:
N A \u003d 12 g / 12 × 1.66057 × 10 -24 g \u003d 1 / 1.66057 × 10 -24 \u003d 6.0221 × 10 -23.
Thus, a mole of a substance contains 6.0221×10 -23 particles of this substance.
The physical quantity N A is called the Avogadro constant, it has the dimension = mol -1. The number 6.0221×10 -23 is called Avogadro's number.
DEFINITION
Molar mass (M) is the mass of 1 mole of a substance.
It is easy to show that the numerical values of the molar mass M and the relative molecular mass M r are equal, however, the first value has the dimension [M] = g/mol, and the second is dimensionless:
M = N A × m (1 molecules) = N A × M r × 1 a.m.u. = (N A ×1 amu) × M r = × M r .
This means that if the mass of a certain molecule is, for example, 44 amu, then the mass of one mole of molecules is 44 g.
Avogadro's constant is a proportionality factor that provides a transition from molecular to molar ratios.
The purpose of the lesson.
P to acquaint students with the concept of "amount of substance", "mole"; form an idea of the molar mass of a substance; teach how to calculate the amount of a substance from a known mass of a substance and the mass of a substance from a known amount of a substance.
Lesson type: a lesson in the study and primary consolidation of knowledge.
Technologies: elements of cooperation technology and problem-based learning.
Methods: heuristic conversation, search activity,
Basic concepts. Amount of substance, mole, Avogadro's number, Avogadro's constant, molar mass.
Planned learning outcomes. Know the number of Avogadro, determine the amount of substance and mole. Be able to determine the number of structural units for a given amount of a substance and vice versa. Be aware of the equality of the numerical values of the molar and relative molecular weights. Be able to calculate the mass of a given amount of a substance.
Equipment: multimedia - equipment, D.I. Mendeleev.
During the classes
1. Organizational moment.
Hello dear guys. My name is Alla Stanislavovna, and today I will give you a chemistry lesson.
My friends! I'm very glad
Sign in to your friendly class
And for me already a reward
Attention of your smart eyes
I know that everyone in the class is a genius
But without labor, talent is not for the future,
Cross the swords of your opinions -
Let's write a lesson together!
2. Statement of the problem of the lesson and goals.
And we will start our lesson with a funny, non-standard situation that once happened in a store.
Eighth grader Kostya went into the store and asked the saleswoman to sell him 10 moles of table salt.. What did the saleswoman say to Kostya?
You will answer this question after studying a new topic.
What term is new to you?
I'll tell you today, or something, about the harmful role of moths.
Moth eats wool and fur - just a panic for everyone ...
Well, in chemistry - if you please! There is another word "mole".
And today in the lesson we will get acquainted with this concept.
Our lesson is called “Amount of substance. Molar mass" ( write in a notebook).
The purpose of our lesson:
firstly: to get acquainted with the concept of "amount of substance", "mole";
secondly: to form an idea of the molar mass of a substance;
thirdly: to learn how to calculate the amount of a substance from a known mass of a substance and the mass of a substance from a known amount of a substance.
3. Learning new material.
Everything is measurable. And you are already familiar with the units of mass or volume. For instance,
When buying sugar, we determine its ___ (weight) with the help of scales, using units of measurement - _______ (kilograms, grams).
When buying bottled milk, we determine its _____ (volume) using measuring utensils, using units of measurement ______ (liter, milliliter)
We can also determine how many pieces (particles) are in 1 kilogram?
Chemistry is the science of substances. Substances are made up of atoms or molecules. In what units can substances be measured? After all, atoms and molecules cannot be counted and weighed.
And then, to measure the substance, a special unit was chosen, in which two quantities were combined - the number of molecules and the mass of the substance.
Such a unit is called the amount of substance or mole.
To measure 1 mole of a substance, you need to take as many grams of it, what is the relative mass of the substance:
1 mol H 2 weighs 2 g (Mr (H 2) \u003d 2)
1 mol O 2 weighs 32 g (Mr (O 2) \u003d 32)
1mol H 2 O weighs 18 g (Mr(H 2 O) = 18)
And how many real particles - molecules are contained in 1 mole of any substance?
It has been established that 1 mole of any substance always contains the same number of molecules. This number is equal to 6 10 23 . For instance,
1 mole of water = 6 . 10 23 H 2 O molecules,
1 mole of iron = 6 . 10 23 Fe atoms,
1 mole of chlorine = 6 . 10 23 Cl 2 molecules,
1 mol of chloride ions Cl - = 6 . 10 23 ions Cl - .
In honor of the Italian scientist Amedeo Avogadro, this number was called Avogadro's constant.
Denoted N A = 6?10 23
The Avogadro constant is so large that it is difficult to imagine.
The Sahara desert contains less than three moles of the finest grains of sand.
If you take 1 mole of dollar bills, they will cover all the continents of the Earth with a 2-kilometer dense layer.
Now we can write down the definition of the concept of "mole".
A MOLE is the AMOUNT of a SUBSTANCE that contains 6 10 23structural units of a given substance -molecules oratoms.
The amount of a substance is indicated by the letter - n, measured in moles
To find out the number of molecules (N), you can use the formula:
knowing the number of molecules, you can find the amount of substance:
And what needs to be done to measure 1 mole of a substance?
You need to take as many grams of this substance as its relative molecular weight.
The mass of 1 mole of a substance is called the molar mass. It is denoted by the letter - M. is found by the formula:
What units will the molar mass be measured in?
measured in (g/mol)
The molar mass coincides in value with the relative atomic or molecular mass, but differ in units of measurement (M - g / mol; Mr, Ar - dimensionless quantities).
M (g/mol) = Mr
The table shows the molar masses for illustration purposes. M for several substances of different structure.
table. Molar masses of various substances.
|
Substance |
Molecular or atomic mass Mr, (Ar) |
Molar mass M |
Avogadro's number |
|
Water H 2 O |
6.02?10 23 molecules |
||
|
Calcium oxide CaO |
6.02?10 23 molecules |
||
|
Carbon 12 C |
6.02?10 23 atoms |
||
|
6.02?10 23 atoms |
|||
|
Chlorine atom Сl |
35.5 g/mol |
6.02?10 23 atoms |
|
|
Chlorine Cl 2 molecule |
6.02?10 23 molecules |
The molar masses of substances differ from each other, but the amount of the substance remains the same - 1 mol.
Number of moles of a substance n found from the mass ratio m of this substance (g) to its molar mass M (g/mol).
Therefore, the mass can be found by the formula:
Let's establish the relationship of the main quantities: m = n ? M, n \u003d m / M, M \u003d m / n, n \u003d N / N A, N \u003d n? N A, where N A 6.02?10 23 mol -1
4. Fixing the material
We have determined how the quantity and mass of a substance are related. Now let's solve the problems using the concepts discussed above.
Task #1 . Determine the mass of oxygen by the amount of substance 3.6 mol.
Task #2 . How much of the substance will contain 64 g of oxygen?
Task #3 . Calculate the amount of substance and the number of molecules contained in carbon dioxide weighing 11 g.
Task 4 . Find the mass of 24 . 10 23 ozone molecules O 3.
Let's try to answer the question asked at the beginning of the lesson:
if the saleswoman studied well in the eighth grade, then she will quickly calculate: mass (? ACl) \u003d 58.5 (g / mol)? 10 (mol) \u003d 585 grams.
After that, she pours salt into the bag, weighs it and politely says "Pay to the cashier."
5. Homework.
So, friends, it's time to say goodbye.
And I want to wish you:
Always eager to learn
Always willing to work.
And never be discouraged.
Literature:
- Alikberova L.Yu. Entertaining chemistry, M, "AST-PRESS", 1999
- Berdonosov S.S., Chemistry 8 class, Miros, 1994.;
- Newspaper "Chemistry at school" No. 44 1996 S. 9.
- Gabrielyan O.S. Chemistry grade 8. M.: Bustard, 2007.
- Ivanova R.G. Chemistry 8-9 grade. M.: Education, 2005.
- Novoshinsky I.I. Novoshinskaya N.S. Types of chemical problems and ways to solve them Grades 8-11. Moscow: Onyx 21st century.
- Educational collection. Chemistry. Basic course. 8-9 grade. Laboratory of multimedia systems of MarSTU. Yoshkar-Ola, 2003.
Try to evaluate your own knowledge and skills after today's lesson
- I understand everything, I can teach others.
- I can explain a new topic with some help.
- I'm having a hard time figuring it out on my own new topic, need help.
|
Formula |
Molar mass (M, g/mol) |
Weight (m, grams) |
Number of particles (N) |
||
|
Formula |
Relative molecular weight (Mr) |
Amount of substance (n, mol)) |
Molar mass (M, g/mol) |
Weight (m, grams) |
Number of particles (N) |
|
Formula |
Relative molecular weight (Mr) |
Amount of substance (n, mol)) |
Molar mass (M, g/mol) |
Weight (m, grams) |
Number of particles (N) |
In chemistry, the concept of "molecular weight" is extremely important. Molecular weight is often confused with molar mass. How do these values differ, and what properties do they have?
Molecular mass
Atoms and molecules are the smallest particles of any chemicals. If you try to express their mass in grams, you get a number in which there will be about 20 zeros before the decimal point. Therefore, it is inconvenient to measure mass in units such as grams. To get out of this situation, some very small mass should be taken as unity, and all other masses should be expressed in relation to it. This unit is 1/12 of the mass of a carbon atom.
Relative molecular weight is the mass of a molecule of a substance, which is measured in atomic mass units. The molecular weight is equal to the ratio of the mass of a molecule of a substance to 1/12 of the mass of a carbon atom. It shows how many times the mass of a molecule of a certain substance is greater than 1/12 of the mass of a carbon atom.
Rice. 1. Table of molecular weights of organic substances.
The atomic mass unit (a.m.u.) is 1.66 * 10 to the -24th degree and represents 1/12 of the mass of a carbon atom, that is, an isotope atom of the carbon element whose mass number is 12. A chemical element in nature can have several stable isotopes, therefore, when talking about the relative atomic mass of an element or, as is often said, about the atomic mass of element A, then the atomic mass of all stable nuclides is necessarily taken into account.
Molecular mass is often confused with molar mass, the unit of which is g/mol. And indeed, numerically, these two quantities are absolutely identical, but their dimensions are absolutely different.
Relative molecular mass can be found by adding together the atomic masses
To calculate the molecular weight of simple and complex substances, it is necessary to find the sum of the relative atomic masses of the atoms that make up the molecule. For example, the relative molecular weight of water Mr (H 2 O), which is known to consist of two hydrogen atoms and one oxygen atom, is 1*2+16=18.
This means that the mass of a water molecule is 18 times greater than 1/12 of the mass of a carbon atom. And the molecular weight of air is 29.
Rice. 2. Formula relative molecular weight.
Atomic mass
The atomic mass of a chemical element is also one of the most important designations in chemistry. Atomic mass is the average value of the atomic masses of the stable natural isotopes of this element, taking into account their relative content in nature (their natural distribution). So, in nature there are two stable isotopes of the element chlorine Cl with mass numbers 35 and 37:
Ar(Cl)=(34.97*0.7553)+(36.95*0.2447)=35.45 - this is the value adopted for the element chlorine as its relative atomic mass.
For the first time, the calculation of atomic weights was made by D. Dalton. He attributed the atomic weights of the elements to the atomic weight of hydrogen, taking it as a unit. However, the weights of the oxygen atom and some other elements calculated in accordance with his principle of "greatest simplicity" turned out to be incorrect.
Rice. 3. D. Dalton.
True atomic masses are minuscule. A hydrogen atom weighs 1.674*10 to -24 grams, oxygen 26.67*10 to -24 grams, and carbon 19.993*10 to -24 grams.
What have we learned?
V school curriculum in chemistry (grade 8), much attention is paid to such concepts as the relative atomic and molecular mass of a substance. Students study their differences and features, and also learn to determine the masses of gases and substances.
Topic quiz
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DEFINITION
The ratio of the mass (m) of a substance to its quantity (n) is called molar mass of the substance:
Molar mass is usually expressed in g/mol, more rarely in kg/kmol. Since one mole of any substance contains the same number of structural units, the molar mass of the substance is proportional to the mass of the corresponding structural unit, i.e. relative atomic mass of a given substance (M r):
where κ is the coefficient of proportionality, the same for all substances. Relative molecular weight is a dimensionless quantity. It is calculated using relative atomic masses chemical elements, indicated in the Periodic system of D.I. Mendeleev.
As you know, the molecular weight of a molecule is equal to the sum of the relative atomic masses of the atoms that make up the molecule:
Mr(HX) = Ar(H) + Ar(X).
M (HX) = Mr(HX) = Ar(H) + Ar(X).
In order to make it easier not to spend time each time calculating the molar mass of a particular substance, a table of molar masses is used, which looks like this:
Examples of problem solving
EXAMPLE 1
| Exercise | Make formulas for two copper oxides if the mass fractions of copper in them are 79.9% and 88.8%. |
| Solution |
ω 1 (O) \u003d 100% - ω 1 (Cu) \u003d 100% - 79.9% \u003d 20.1%; ω 2 (O) \u003d 100% - ω 2 (Cu) \u003d 100% - 88.8% \u003d 11.2%. Let us denote the number of moles of elements that make up the compound as "x" (copper) and "y" (oxygen). Then, the molar ratio will look like this (the values of relative atomic masses taken from the Periodic Table of D.I. Mendeleev are rounded to whole numbers): x:y = ω 1 (Cu)/Ar(Cu) : ω 1 (O)/Ar(O); x:y = 79.9/64: 20.1/16; x:y = 1.25: 1.25 = 1: 1. So the formula of the first copper oxide will be CuO. x:y = ω 2 (Cu)/Ar(Cu) : ω 2 (O)/Ar(O); x:y = 88.8/64: 11.2/16; x:y = 1.39: 0.7 = 2: 1. So the formula of the second copper oxide will be Cu 2 O. |
| Answer | CuO and Cu 2 O |
EXAMPLE 2
| Exercise | Make formulas for two iron oxides if the mass fractions of iron in them are 77.8% and 70.0%. |
| Solution | The mass fraction of the element X in the molecule of the HX composition is calculated by the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Find the mass fraction in each of the copper oxides: ω 1 (O) \u003d 100% - ω 1 (Fe) \u003d 100% - 77.8% \u003d 22.2%; ω 2 (O) \u003d 100% - ω 2 (Fe) \u003d 100% - 70.0% \u003d 30.0%. Let us denote the number of moles of elements that make up the compound as "x" (iron) and "y" (oxygen). Then, the molar ratio will look like this (the values of relative atomic masses taken from the Periodic Table of D.I. Mendeleev are rounded to whole numbers): x:y \u003d ω 1 (Fe) / Ar (Fe) : ω 1 (O) / Ar (O); x:y = 77.8/56: 22.2/16; x:y = 1.39: 1.39 = 1: 1. So the formula of the first iron oxide will be FeO. x:y \u003d ω 2 (Fe) / Ar (Fe) : ω 2 (O) / Ar (O); x:y = 70/56: 30/16; x:y = 1.25: 1.875 = 1: 1.5 = 2: 3. So the formula of the second iron oxide will be Fe 2 O 3 . |
| Answer | FeO, Fe 2 O 3 |
