How to calculate the mass fraction of an element in a substance. Solving problems on the determination of chemical formulas by mass fractions of the elements that make up the substance
Lesson17
Theme. Calculation of the mass fraction of the element in the composition of the substance.
Solving problems on the determination of chemical formulas by mass fractions of the elements that make up the substance
Objectives: to deepen students' knowledge of the mass fraction of the element in the composition of the substance; improving practical skills for calculating the mass fraction of a substance; solving problems of determining the formula of a substance by the mass fractions of the elements that make up the substance molecule.
Type of lesson: the deepening and adjustment of knowledge, consolidation of practical skills.
Forms of work: frontal work, written independent work.
Equipment: periodic system of chemical elements, cards with tasks for frontal work, algorithm.
Ii. Knowledge update
1. A student writes on a blackboard and explains the formula for the mass fraction of an element in the composition of a substance.
2. Check homework (oral frontal survey and adjustments).
3. Calculate the mass fractions of the elements in the H 2 SO 4 molecule.
Iii. Consolidating skills
Training exercises
Task 1. Arrange the substances in ascending order of the mass fraction of Oxygen in the composition of the molecule. (Fill the table on the board.)
Cu2O |
Al2O3 |
Fe2O3 |
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20,7 |
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Task 2. (Explanation of the teacher.) Some connection of Nitrogen with Oxy - gene contains 63.2% of Nitrogen. Determine the formula of this substance, if its molecular weight is 76 a.Am.
Therefore, the number of oxygen atoms is three.
2. m (N) = 76 - 3 · 18 = 28.
Knowing that Ar (N) = 14,
in = 28/14 = 2.
Therefore, the number of Nitrogen atoms is two, and the formula of the substance is N 2 O 3.
Task 3. Mass fraction of Carbon in its connection with Oxygen - 27.3%. Determine the formula of this compound, knowing that its relative molecular weight is 44.
Answer: CO 2.
Task 4. The mass fraction of Cuprum in conjunction with Oxygen is 88.9%. The relative molecular weight of this compound is 144. Determine the formula of the compound CuxOy.
Answer: Cu 2O.
Iv. Written independent work on options
Task 1. According to the chemical formula of a substance:
Describe the qualitative composition of the molecule;
Describe the quantitative composition of the molecule;
Calculate the mass fractions of the elements in the composition of the molecule of this substance.
Option I |
Option II |
Option III |
FeCl3 |
P2O5 |
C3H8 |
Cu (OH) 2 |
Fe (OH) 3 |
Al (OH) 3 |
Na2SO4 |
K2CO3 |
Na3PO4 |
Task 2. For a known mass fraction of Chlorine in the composition of the substance and atomic molecular weight, derive the formula for this substance.
Mr (MgxCl in) = 95 |
Mr (AlxCl in) = 133.5 |
Mr (FexCl in) = 162.5 |
ω (Cl) = 74.7% |
ω (Cl) = 79% |
ω (Cl) = 65% |
V. Homework
Repeat the section "Mass fraction of the element in the composition of the substance."
Creative task. The composition of the baking soda molecule includes atoms of sodium, hydrogen, carbon and oxygen. Mass fractions of elements are respectively equal: ω (Na) = 27.4%; ω (H) = 1.2%; ω (C) = 14.3%; ω (O) = 57.1%. Determine the formula of baking soda and calculate its relative molecular weight.
LESSON THEME: Mass fraction of the chemical element in the compound.
THE PURPOSE OF THE LESSON: To teach how to calculate the mass fraction of elements in a compound using the compound formula and establish the chemical formula of a complex substance based on the known mass fractions of chemical elements.
Basic concepts. Mass fraction of the chemical element.
Planned learning outcomes
Subject. To be able to calculate the mass fraction of an element in a compound by its formula and establish the chemical formula of a complex substance by the known mass fractions of chemical elements.
Meta-Subject . To form the ability to establish analogies, use algorithms for solving educational and cognitive tasks.
The main activities of students. Calculate the mass fraction of an element in a compound by its formula. To establish the chemical formula of a complex substance by the known mass fractions of chemical elements.
Lesson structure
I. Organizational stage
II. Updating reference knowledge
III. Learning new material
IV. Anchorage. Lesson Summary
V. Homework
During the classes
Organizing time.
Check homework.
Updating reference knowledge.
Give a definition: relative atomic mass, relative molecular weight.
In what units can the relative atomic mass be measured.
In which units can the relative molecular weight be measured.
The study of new material.
Work with the textbook. Workbook.
Guys, let's say we have a substance - sulfuric acidH 2 SO 4,
can we find out which atoms are part of a compound?
And their number?
And in what mass ratio do they connect?
Calculating chemical mass ratios
elements in a complex substance. (p. 51)
And how can you find out in which mass relations elements are combined in a compound whose formulaH 2 SO 4 ?
m(H): m(S): m(O)= 2*2 + 32 + 16*4= 2:32:64 = 1:16:32.
1 + 16 + 32 = 49, that is, 49 mass parts of sulfuric acid, 1 mass part of hydrogen, 16 mass parts of sulfur, 32 mass parts of oxygen are contained.
Guys, what do you think, can we calculate the proportion of each element in the mix?
Today we will get to know the new concept of the mass fraction of an element in a compound.
W - mass fraction of the element in the compound.
n - the number of atoms of the element.
Mr - relative molecular weight.
Calculation of the mass fractions of chemical elements
in a complex substance. (RT)
1. Learn the algorithm for calculating the mass fraction of an element in a compound.
Task number 1 (RT)
The derivation of chemical formulas, if known mass fractions of chemical elements,
included in the composition of the substance. (RT)
2. Learn the algorithm for calculating the mass fraction of an element in a compound.
Task number 5 (RT)
Consolidation of the material studied.
RT page 25 №2.
RT p. 27 №6.
Summing up the lesson.
What new concepts have you learned today in class?
Independent work.
Homework:
study §15 pp. 51 - 53;
answer questions № 3,4,7 pp. 53-54 (in writing).
P the list of used literature.
Textbook. Chemistry grade 8. auth. G.E. Rudzitis, F.G. Feldman. Publisher "Enlightenment", 2014.
Workbook in chemistry. auth. Borovskikh T.A.
Chemistry lesson in grade 8
"Mass fraction of an element in a substance"
Theme of the lesson:Mass fraction of an element in a substance
Purpose:withform the concept of “mass fraction of an element” in students; develop skills in finding the mass fraction of an element in a chemical compound; to improve the skill of finding the relative molecular weight, to characterize the qualitative and quantitative composition of the substance.
During the classes
Epigraph to the lesson:
Do not let the soul be lazy
So as not to crush water in a mortar,
The soul must work
Both day and night
Both day and night!
(M.Zabolotsky).
1. Organizational moment. Positive attitude.
2. Motivation.
The components of our planet are organic and inorganic world, formed by organic and inorganic substances. Look at the globe. ¾ the surface of our planet is occupied by matter, without which there would be no life on our earth. What is this substance? (water)
You already know that any substance can be written using a chemical formula. Recall the formula of water? What can you determine from this formula?
(qualitative and quantitative composition, simple or complex substance, determine what kind of substance, find М r).
Guys, what elements are part of the water molecule? And how many atoms of each element is contained in a water molecule? And if you change the number of atoms of oxygen or hydrogen in a water molecule, will the changed formula belong to the substance water (students' answers)?
And so, the atoms of chemical elements are in a molecule in certain mass relations, i.e. make up a certain part of the total molecule. And how can you calculate - what part of the molecule is a chemical element (students' suggestions)?
You have a certain baggage of knowledge, what concepts, studied earlier will help you in achieving the goal (" conceptual chamomile "(student activity - 1 point) - chemical element, chemical formula, relative atomic and relative molecular weight, fraction)? What term seemed unfamiliar to you? So, what do you think we will talk about today?
3. Announcement of the lesson topic.
With your help, we have identified the topic of our lesson:
“Mass fraction of an element in a substance”.
What do you think, what is the purpose of our lesson, what do we want to learn ... and what to learn ... (students determine the purpose of the lesson)?
4. Actualization of supporting knowledge.
Check whether you are ready to learn new material.
Will hold chemical baton on the knowledge of the symbols of the elements (the students in the group verify that the assignment is done correctly with the board (2 points are given to each student in the series if there are no errors, the additional point is the series that coped with the task first (if there are no errors) and 0 points (not knowing chemical signs). The result is announced
Give a qualitative and quantitative characterization of substances.
Find the relative molecular weight
The second task (4 points) is performed individually (two at the board).
We carry out mutual verification of the completed task. With the right solution of the problem, you get 2 points for the task, if you have a mathematical error or correction - 1 point each, the wrong decision - 0 points.
(between results- Who scored 6 points, 5 points, 4 points?)
5. Learning new material.
We repeated the necessary definitions, chemical elements, molecular weight, and what does the term share mean? We repeat this concept, verbally solving math problems on finding a fraction.
Problem number 1
In the box cubes and balls. What is the share of cubes in a box? (students decide and compare with ready-made solution)
Solution: only 6 items, cubes 2.
Their share is 2/6.
Task number 2 -complicate the task
The cube weighs 10 mg, the bead weighs 20 mg. What is the proportion of cubes in the box?
Solution: each cube weighs 10 mg,
and only two of them, then the total mass 20 mg.
Balls 4, each weighs 20 mg, i.e. in total - 80 mg.
All items have weight - 100 mg.
The total weight of 100 mg, then the proportion of cubes - 20/100.
As a result, we derive the formula for calculating the share -W = m (parts) /m (general)
All students actively involved in solving problems add a 1 point.
“FIZKULT HELLO!” (Conducting a video clip)
As a result of the previous assignment, we got the formula with you -
W = m (parts) / m (total)
And now, let's work with the textbook - (the notion of mass fraction) p. ___ paragraph __ (read the wording). We are familiar with the formula for calculating the mass fraction of an element in a substance.
We make the "Associative Bush"
6. Fixing.
Moving from theory to practice. Consider the scheme of the structure of the water molecule. You can find the mass of atoms in the table of chemical elements of D.I. Mendeleev; we will find the fraction of elements in the composition of water.
Rules for the task (a student draws up a task design template with the help of a teacher).
We analyze the oral solution - So, what is the mass fraction of oxygen and hydrogen? Try to solve the problem yourself. (work on options)
Given: Solution:
H 2 O Mr (H 2 O) =
Find W (O) =
Check by solution stages: Mr = 18, W (O) = 0.89 or 89%, W (H) = 0.11 or 11%
Why does a person need knowledge of the mass fraction? Consider an example.
You and I live in Crimea, one of the problems of the population of the peninsula is the lack of iodine in food. "Endemic goiter" - the result of such a shortage. It is therefore necessary to eat iodized salt. Foods are enriched with iodine using compounds such as potassium iodide and potassium iodate. Knowing the concept of the mass fraction of an element in a compound, find out which of the substances contains more iodine?
The correct decision - 2 points, inaccurate, with corrections - 1 b, incorrect - 0b
7. Summing up the lesson. Reflection. Guys, how do you think, why did I choose the words of the Russian writer M. Zabolotsky as an epigraph to the lesson?
You're right! There are no people to whom knowledge comes themselves, only systematic work raises them every day a step higher. If you are not satisfied with yourself today - do not be upset, you will work at home on your own, and tomorrow you will succeed.
8. Homework.
For all students - to study §textbook, to compose and solve the problem on the studied topic.
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