Topic: How to calculate molecular weight

Target group

Elementary school student (grades 7. and 8.)

Core curriculum

Elementary school. Chemistry.

III. Chemical reactions. Pupil:

6) calculates molecular masses of elements occurring in the form of molecules and chemical compounds.

General aim of education

The student calculates molecular masses

Key competences

• communication in the mother tongue;

• communication in foreign languages;

• mathematical competence and basic competences in science and technology;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• calculate the molecular mass of simple chemical compounds;

• determine the mass ratios of elements in chemical compounds;

• determine the percentage composition of the chemical compound.

Methods/techniques

• expository

  • talk.

• activating

  • discussion.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• activity in groups;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers;

• methodician or green, yellow and red cards;

• periodic table of elements.

Lesson plan overview

Introduction

1. The teacher hands out Methodology Guide or green, yellow and red sheets of paper to the students to be used during the work based on a traffic light technique. He presents the aims of the lesson in the student's language on a multimedia presentation and discusses the criteria of success (aims of the lesson and success criteria can be send to students via e‑mail or posted on Facebook, so that students will be able to manage their portfolio).

2. The teacher together with the students determines the topic – based on the previously presented lesson aims – and then writes it on the interactive whiteboard/blackboard. Students write the topic in the notebook.

Realization

1. The teacher, introducing students to the subject of classes, discusses the concept of mass of an atom. He gives her the approximate value and the unit in which it is expressed. Informs that atomic masses are rounded to whole numbers - except for chlorine (35.5 u) and copper (63.5 u). He explains why atoms of the same element can differ from each other by mass. He emphasizes that the atomic mass of the element in the periodic table is the average atomic mass and is a characteristic feature of a given element, after which it can be identified. Working with the active element system, students practice the reading of atomic masses.

2. The teacher discusses the rules for calculating molecular masses of elements and chemical compounds. He explains what molecular weight is.

3. The lecturer explains when the concept of molecular weight of chemical elements should be used - it presents the method of calculating the molecular weights of some elements, eg hydrogen - HIndeks dolny 2₂, oxygen - OIndeks dolny 2₂, nitrogen - NIndeks dolny 2₂, ozone - OIndeks dolny 3₃, phosphorus - PIndeks dolny 4₄, sulfur - SIndeks dolny 8₈, chlorine - ClIndeks dolny 2₂, bromine - BrIndeks dolny 2₂, fluorine - FIndeks dolny 2₂, iodine - IIndeks dolny 2₂. After discussing, for example, 2‑3 examples, he asks those willing to the table to solve further tasks related to the calculation of molecular masses of chemical elements. Other students perform calculations in notebooks.

4. The teacher displays on the multimedia board the presentation „Molecular weights of selected chemical compounds” from abstract and together with students they analyze several examples. Then the students write their observations and conclusions on the form.

5. The teacher gives examples of chemical compounds - those who are willing to approach the table and calculate molecular masses. As examples, the teacher may suggest: HIndeks dolny 2₂SOIndeks dolny 3₃, HIndeks dolny 2₂S, HIndeks dolny 2₂COIndeks dolny 3₃, HNOIndeks dolny 3₃, Fe(OH)Indeks dolny 3₃, Cu(OH)Indeks dolny 2₂, NHIndeks dolny 3₃

6. At the end of the lesson, the teacher asks students to do an interactive exercise - individual work.

Summary

1. The teacher asks the students to finish the following sentences:

   • Today I learned ...

   • I understood that …

   • It surprised me …

   • I found out ...

   The teacher can use the interactive whiteboard in the abstract or instruct students to work with it

Homework

1. Listen to the abstract recording at home. Pay attention to pronunciation, accent and intonation. Learn to pronounce the words learned during the lesson.

2. Make at home a note from the lesson using the sketchnoting method.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

How to calculate molecular weight

The atomic weight is very small and constitutes only a small part of gram (approx. 10Indeks górny -24^-24 g). That is why it is expressed using atomic mass units, called the units. One unit is equal to 0.00000000000000000000000166 g (1.66 · 10Indeks górny -24^-24 g).

Atoms of the same element may differ in their weight as there are certain varieties with different number of neutrons in nucleus. That is why atomic weight of an element is the weighted average of its isotopic masses. In order to calculate it, the percentage of isotopes of a given element is taken into account. The average atomic weights of elements are listed in the periodic table.

The atomic weight is a unique feature of given element and there are no two different elements with the same atomic weight. That is why if you know atomic weight, you can identify the element.

Chemical compounds are made of molecules or form crystal structure with ions in a specific quantitative ratio. Molecular formula of a compound shows molecular structure of a molecule or – if it is an ionic compound – it corresponds to the simplest stoichiometric ratio of ions present in this compound. The mass of these smallest structures of chemical compounds, expressed using atomic mass units, is called molecular weight. Although the term consists of a “molecular” part, its scope is not limited only to molecular compounds but also covers ionic compounds.

The molecular weight of a chemical compound is equal to the sum of atomic weights of elements forming the smallest structure of a chemical compound that is described using the chemical formula.

$\mathrm{atomic\; weight\; of\; chlorine}= 35.5\mathrm{ u}$

$\mathrm{atomic\; weight\; of\; copper}= 63.5\mathrm{ u}$

When making approximations, you have to follow certain rules:

If the first decimal digit is less than 5, all decimal digits are rejected, for example:

$\mathrm{atomic\; weight\; of\; magnesium}= 24.31\mathrm{ u }\approx 24\mathrm{ u}$

If the first decimal digit is greater than 5, the number corresponding to a unit is increased by 1, for example:

$\mathrm{atomic\; weight\; of\; lithium}=6.94u\approx 7u$

In case of chlorine and copper the first decimal digit is equal to 5 (their atomic weights are not rounded up to integers).

Some chemical elements, such as: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine or iodine, occur in the form of diatomic molecules. In order to determine their weight, we use molecular weight of an element. The molecular weight of a chemical element is a product of its atomic weight and a number of atoms that make up a molecule.
For example, the molecular weight of nitrogen is made up of diatomic molecules with the formula ${\text{N}}_{\text{2}}$ is as follows:

$\text{2 · atomic weight of nitrogen = 2 · 14 u = 28 u}$

The ratio of the number of atoms (or ions) of individual elements in given compound is determined based on molecular formula of a chemical compound. For example, in hydrogen chloride with the following molecular formula: $\text{HCl}$ there is one atom of hydrogen per one atom of chlorine. The ratio of hydrogen atoms to chlorine atoms is 1 : 1. In case of water with molecular formula ${\text{H}}_{\text{2}}\text{O}$, the ratio of hydrogen atoms to oxygen atoms is 2 : 1.

Using the term “atom” to describe the numerical ratio would not be a big mistake, regardless of the compound type (covalent or ionic one).

A molecular formula of a chemical compound contains information about elements that form this compound and quantitative ratio in which they bond with each other. If you know these information, it is easy to calculate mass ratio of individual elements. This value is defined as the ratio of atomic weights of individual elements that form given chemical compound. In a hydrogen chloride with a formula $\text{HCl}$ there is one atom of hydrogen with atomic weight of 1 u and one atom of chlorine with atomic weight of 35.5 u. The mass ratio of hydrogen to chlorine in a hydrogen chloride molecule amounts to 1 : 35.5.

In a compound with general formula:

${\text{A}}_{\text{x}}{\text{B}}_{\text{y}}$

the mass ratio is as follows:

$\text{x · atomic weight of element A : y · atomic weight of element B}$

• Molecular weight, mass ratio of individual chemical elements and percentage content of these elements can be calculated based on formula of given chemical compound.

• Molecular weight of chemical compound is equal to the total weight of atoms of elements making up the molecule (if it is a covalent compound) or the smallest set of repetitive ions (if it is an ionic compound). It is expressed using atomic mass units.

• The mass ratio of individual elements making up a chemical compound is called mass ratio.