Topic: How to calculate molecular weight – percentage composition

Target group

Elementary school student (grades 7. and 8.)

Core curriculum

Elementrary school. Chemistry.

III. Chemical reactions. Student:

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

General aim of education

The student calculates the percentage of elements in the chemical compound and mass ratios, knowing the percentage composition of the elements that make up the chemical compound.

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:

• to calculate mass ratios, knowing the percentage composition of elements that make up a chemical compound;

• to determine the percentage composition of the chemical compound.

Methods/techniques

• expository

  • talk.

• activating

  • discussion.

• exposing

  • film.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers.

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. Students get acquainted with the content of the abstract and define the concept of the percentage of an element in a chemical compound.

2. The teacher plays and discusses the presentation titled „How to calculate percentage composition of elements in a chemical compound?” regarding sodium oxide (NaIndeks dolny 2₂O) and phosphoric acid (HIndeks dolny 3₃POIndeks dolny 4₄).

3. Students, working in pairs, analyze examples presented in the table „Percentage composition of oxygen in selected chemical compounds”. The pointed out couples discuss their within the class.

4. Students carry out exercise number 1: they calculate percentage composition (mass fraction) of chemical elements in calcium chloride if the mass ratio of calcium to chlorine in calcium chloride amounts to 4 : 7.

5. The teacher plays the presentation titled „Calculation of mass fraction if percentage composition of elements forming chemical compound is known” on the example of ethane percentage composition: 80% C and 20% H.

6. Volunteers solve the tasks on the board: a) calculate the mass ratio of nitrogen to oxygen in nitric oxide, knowing that nitrogen is 26% nitrogen and 74% oxygen; b) calculate the mass ratio of sulfur to oxygen in sulfur dioxide, knowing that in sulfur it is 50%..

7. Students carry out the interactive exercises checking the level of knowledge learned during the lesson. The teacher initiates a discussion during which the correct solutions for all the exercises performed by the students are discussed.

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.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

How to calculate molecular weight – percentage composition

Percentage of an element in a chemical compound is calculated using the following formula:

$\text{mass percent of given element =}\frac{\text{mass of given element in the smallest structure of chemical compound [u]}}{\text{molecular weight of chemical compound [u]}}\text{ · 100 \%}$

where:

$\text{mass of given element in the smallest structure = number of element’s atoms in the formula · atomic weight of an element [u]}$

Percentage content of a chemical element in a chemical compound is a ratio of weight of atoms of this element (expressed in units) in the smallest part of the chemical compound to the molecular weight of the compound, multiplied by 100%. A molecule is considered the smallest part of a chemical compound in case of covalent compounds, while the smallest set of repetitive ions is the smallest part in ionic compounds.

The percentage content is mass ratio of given element in total mass of a chemical element. That is why this value is sometimes called percentage mass content. If percentages of all elements present in a compound, we refer to percentage composition of the chemical compound.

The total percentage of all elements in the chemical compound is always equal to 100%. This means that without knowing the percentage of one element that is part of the compound one can easily calculate this value based on percentage for other elements of given compound.

• Mass ratio of an element in a chemical composition is called the percentage content. It can be calculated from the the mass of given element in the smallest structure of the chemical compound to the molecular weight multiplied by 100%. Molecules are the smallest elements of a covalent compound, while the smallest set of repetitive ions is the smallest part in ionic compounds.