Topic: Percent concentration of solution - preparation of solutions in practice

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

V. Water and aqueous solutions. Pupil:

7) performs calculations using the following notions: solubility, percent concentration (mass percentage), mass of substance, solvent mass, mass of solution, solution density (using solubility table or solubility plot).

General aim of education

The student solves the text assignments for the percentage concentration of the solution

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 percentage concentration of a solution based on its mass or mass of solvent and mass of solute;

• estimate the mass of the solution based on the percentage of solution and the mass of solute;

• determine the mass of solute based on the percentage and mass of the solution.

Methods/techniques

• activating

  • discussion.

• expository

  • talk.

• exposing

  • film.

• 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;

• projector;

• interactive whiteboard, tablets/computers;

• methodician or green, yellow and red cards;

• equipment, laboratory glass and reagents for observation.

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.

3. Health and safety – before starting the experiments, students familiarise themselves with the safety data sheets of the substances that will be used during the lesson. The teacher points out the need to be careful when working with them.

Realization

1. The teacher displays on the multimedia board a presentation „Stages of solving the task” presenting a phased solution of the task.

2. The teacher divides the students into groups and informs them that they will perform the experiment according to the instructions described in the abstract. Can each group indicate other solutions for preparation, giving quantities, e.g. 130 g of a 5% solution; 140 g of a 3% solution.

3. The lecturer gives the students successive tasks to solve, asking those willing to the board (examples of tasks described in the methodical commentary).

4. Students, working individually or in pairs, carry out interactive exercises to check and consolidate knowledge learned during the lesson. Selected people discuss the correct solutions for interactive exercises. The teacher completes or straightens the statements of the proteges.

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

2. The student indicated by the teacher sums up the lesson, telling what he has learned and what skills he/she has been practicing.

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.

Methodical Comment

Examples of tasks to be solved during the lesson:

Excercise 1.

How many grams of table salt and how much water should you take to get 300 g of 5% solution?

Data:
mIndeks dolny r_r – 300g
CIndeks dolny p_p – 5%

Unknown:
mIndeks dolny w_w – ?
mIndeks dolny s_s – ?

${\mathrm{C}}_{\mathrm{p}}=\frac{{\mathrm{m}}_{\mathrm{s}}}{{\mathrm{m}}_{\mathrm{r}}}\times 100\%$

${\mathrm{m}}_{\mathrm{s}}=\frac{{\mathrm{C}}_{\mathrm{p}}\times {\mathrm{m}}_{\mathrm{r}}}{100\%}=\frac{5\%\times 300\mathrm{g}}{100\%}=15\mathrm{g}$

mIndeks dolny w_w = mIndeks dolny r_r - mIndeks dolny s_s = 300g – 15g = 285g

Answer: To get 300g of a 5% solution, you need to take 285g of water and 15g of salt.

Exercise 2.

How many grams of the substance should be dissolved in 400 g of water to get a 20% solution?

Data:
mIndeks dolny w_w – 400g
CIndeks dolny p_p – 20%

Unknown:
mIndeks dolny s_s – ?

100g of solution contains 20 g of substance
(400g + X) solution contains Xg of substance

(400g + X) x 20g = 100g x X
8000gIndeks górny 2² + 20g x X = 100g x X
20g x X - 100g x X = -8000 gIndeks górny 2²
X(20g - 100g) = -8000 gIndeks górny 2²

$\mathrm{X}=\frac{-8000{\mathrm{g}}^2}{\left(20\mathrm{g}-100\mathrm{g}\right)}=\frac{-8000{\mathrm{g}}^2}{-80\mathrm{g}}=100\mathrm{g}$

Answer: To obtain a 20% solution, dissolve 100 g of the substance in 400 g of water.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Percent concentration of solution - preparation of solutions in practice

To prepare a solution with a certain concentration, you need to know the mass of its constituents: solvent and solute. For this purpose, appropriate calculations must be made in advance. Then individual substances are weighed and mixed together.

To prepare 150 g of a 2% aqueous sodium chloride solution, measure the appropriate volume of water and weigh the appropriate amount of sodium chloride (see table below).

Using a measuring cylinder, measure 147 cmIndeks górny 3³ of water and pour it into the beaker. Weigh 3 g of sodium chloride on the scale and dissolve it in water.

• Percent concentration is the information about how many parts of the mass (in weight) of the dissolved substance are found in 100 parts of the mass (in weight) of the solution.

• The percent concentration of a solution can be calculated based on the weight of the solution and the weight of the solute, using the formula: ${\text{c}}_{\text{p}}\text{=}\frac{{\text{m}}_{\text{s}}}{{\text{m}}_{\text{r}}}\text{· 100\%}$.

• The mass of solute can be calculated based on the percent concentration and mass of the solution: ${\text{m}}_{\text{s}}\text{=}\frac{{\text{c}}_{\text{p}}·{\text{m}}_{\text{r}}}{\text{100\%}}$.

• The percent concentration of the solution can be calculated based on the weight of the solvent and the weight of the solute, using the formula: ${\text{c}}_{\text{p}}\text{=}\frac{{\text{m}}_{\text{s}}}{{\text{m}}_{\text{solv.}}\text{+}{\text{m}}_{\text{s}}}\text{· 100\%}$.

• If the density of the solution is known, it is possible to determine the mass of given volume.