0911 Concentrated, dilute, strong, weak

10/12/2020

Module 0911

Concentrated, dilute, strong, weak

Does the word concentrated refer to solutions, or to solutes?

And the term dilute?

What about strong, and weak?

Some people use these terms loosely. And wonder why there is confusion!

Preface: It's all about language

Precise use of language can assist understanding. Careless language can cause confusion.

In Chemistry, there are terms agreed upon by IUPAC. Particular terms have particular meanings, and we should use them correctly or the receiver of a communication (a listener, or a reader) may become confused, or even develop a wrong understanding of the idea being discussed.

Unfortunately, over the years IUPAC has decided to use terms which have different meaning in everyday life from their meaning in Chemistry. This can be confusing if we don’t recognise which “compartment” (Chemistry, or everyday) that we are operating in.

The terms concentrated, dilute, strong, and weak are often used wrongly, because of either lack of care, or ignorance.

Clarification awaits in this video ..... Aussie is still away.

Prof Bob sets straight the correct usage of the terms concentrated, dilute, strong, and weak

KEY IDEAS - Concentrated, dilute, strong, and weak

On the other side of the bottle is a label that reads "Strong sulfuric acid". Incorrect, and misleading! Why?

Do these terms apply to solutions or to solutes?

Among the terms concentrated, dilute, strong, and weak, there are two related pairs:

Concentrated and dilute are opposite ends of the scale of a property (concentration) of SOLUTIONS.
Strong and weak are opposite ends of the scale of a property (extent of ionisation) of SOLUTES.

Solutions

The concentration of a solutions refers to how crowded the solute species are in the solution – regardless of the volume of solution. See Module 0907 Solution concentration.

We use the following terms:

The term concentrated solution refers to solutions with high concentration of solutes (The solute species are very crowded).
The term dilute solution refers to solutions with low concentration of solutes (The solute species are widely spaced, on average).

There is no sharp dividing line between concentrations that we refer to as either concentrated or dilute – although it is common to use these terms for solutions whose concentrations are near either extreme.

Mostly, these terms are used in a relative sense, such as:

A 2 M solution of sucrose in water is more concentrated than a 0.1 M solution of sucrose.
A 0.001 M solution of sucrose is more dilute (or less concentrated) than a 0.01 M sucrose solution.

Solutes

The terms strong and weak are opposite ends on the scale of extent of ionisation of solutes that are electrolytes (i.e., when the solute dissolves in water, some of the molecules break up into ions). Refer to the previous Module 0909 Solutes: Electrolytes or non-electrolytes?

And a reminder from a previous Module 0910 Electrolytes: Strong or weak?

A solute which is a strong electrolyte: On dissolving in water, all of the solute molecules break up into ions.
A solute which is a weak electrolyte: On dissolving in water, only some of the solute molecules break up into ions.

Again, these terms are often used in a comparative sense.

Here are some examples that show the distinctly different meanings of the two pairs of words: concentrated - dilute, and strong - weak.

10 M HCl solution: A rather concentrated solution of a strong electrolyte.
0.001 M HCl solution: A rather dilute solution of a strong electrolyte.
5.0 M solution of acetic acid: A rather concentrated solution of a weak electrolyte.
0.01 M solution of formic acid: A rather dilute solution of a weak electrolyte
5.0 M solution of sucrose: A rather concentrated solution of a non-electrolyte.

In summary:

Solutions are more or less concentrated (or dilute).
Solutes are more or less strong (or weak) electrolytes.

So, whenever we use any of these four terms, we should think “Am I talking about the solution, or about the solute?”
It will assist your understanding.

Language, language, .......

If you would like to contemplate a little deeper about the importance of language, and how people communicating need to have matched meaning for words, do go to Module 2703 The jargon we use. Discussion can be not profitable, and/or confusing, if (i) the meanings of terms are different for different people, or (ii) one party is ignorant altogether of the meanings of the terms used.

SELF CHECK: Some thinking tasks

SELF CHECK - Some thinking tasks

Get it right, now. Recovery of the pathway can be difficult.

1. The following six diagrams are symbolic portrayals, at the level of molecules and ions, of:

A concentrated solution of a strong electrolyte solute
A dilute solution of a strong electrolyte solute
A concentrated solution of a weak electrolyte solute
A dilute solution of a weak electrolyte solute
A concentrated solution of a non-electrolyte solute.
A dilute solution of a non-electrolyte solute.

Water molecules are not shown in the representations.
Decode the symbols for (i) solute molecules, (ii) cations, and (iii) anions, and then decide which of the solutions is represented by each of the diagrams.

2. This questions involves comparison of the three solutions A, B, and C, listed here:
            A         0.20 M aqueous sucrose solution
            B 0.05 M aqueous sucrose solution
            C 0.01 M aqueous sucrose solution

(a)        Which one of these solutions is the strongest? Which is the weakest?
(b)        Make some valid statements about the relative concentrations of the solutions A, B, and C.

3. Consider the following three aqueous solutions of hypothetical weak electrolytes HX, HY, and HZ.
A. 0.10 M solution of HA. 11% of the HX molecules are ionised, and 89% remain un-ionised.
B. 0.10 M solution of HB. 4% of the HY molecules are ionised, and 96% remain un-ionised.
C. 0.10 M solution of HC. 0.5% of the HZ molecules are ionised, and 99.5 % remain un-ionised.

(a) Make some valid statements about the relative strengths of the solutes HX, HY, and HZ.
(b) Why would your ability to answer question (a) be more uncertain if all of the solutions were not 0.10 M?

4. Comment on what this person said: “I’m using quite a strong acetic acid solution – it’s 5.0 M. I think that I should use a weaker solution – like 0.01 M”.

5. Consider the following solutions:
Insert appropriate words in the spaces in the following sentences related to the two aqueous solutions A and B:
A: 0.01 M hydrochloric acid (HCl) solution
B: 0.08 M acetic acid (CH3COOH) solution.
C: 0.05 M sucrose (C12H22O11) solution
Insert appropriate terms in the spaces of the text below. Choose from the following words and phrases:

concentrated
more concentrated
less concentrated
dilute
more dilute
less dilute
strong
stronger
weak
weaker
electrolyte
strong electrolyte
weak electrolyte
non-electrolyte.

Solution A is more ……...…..…… (that is, less ……........……) than solution B. Solution B is ………......….. than solution C. The solute in solution A is a ………....…...………. electrolyte than the solute in solution B. In fact, because all of the HCl molecules ionise in solution, hydrochloric acid is a …………..…………. . The solute in solution B is a ……....…..…….. because only some of the molecules ……..........… in solution, but it is a ……..…........…… than the solute in solution C. In solution C, none of the sucrose molecules ionise, so sucrose is called a ….......…....…. .

6. Relational (or Venn) diagrams show the interrelationships among sets of objects or ideas. For example, a relational diagram can portray the relationships such as the following:

In one set (category) of dogs (A), all are black. But not all dogs are black.
In another set (B), all the dogs have short tails
In another set (C), all the dogs have long hair
A subset of the dogs in set A and a subset of those in set B have the common properties of being black with short tails
A subset of the dogs in set A and a subset of those in set C have the common properties of being black with long hair.
A subset of the dogs in set B and a subset of those in set C have the common properties of having short tails and long hair.
A subset of the three subsets have the common properties of blackness, short tails and long hair.

(a) Draw a relational (Venn) diagram that shows the interrelationships between the following concepts:

Solutions of an electrolyte
Solutions of a strong electrolyte
Solutions of a weak electrolyte
Solutions of a non-electrolyte
Concentrated solutions
Dilute solutions

You might, for example, begin with trying to portray diagrammatically that every aqueous solution is either (i) a solutions of an electrolyte, or (ii) a solution of a non-electrolyte.

(b) Put each of the following solutions in the appropriate area of the diagram. That is, consider in which area of the diagram each solution “belongs”.

vodka (37% ethanol in water)
300 g/L NaCl solution
50% aqueous ethanoic acid solution
M sucrose solution
M copper sulfate (CuSO4) solution
10 M HCl solution
M ammonia (NH3) solution
18 M sulfuric acid (H2SO4) solution
M HCl solution
vinegar (5% ethanoic acid in water)
5 M ammonia (NH3) solution

Remember that there is no single concentration that is a sharp dividing line between solutions that are classified as “concentrated” or “dilute”. Nevertheless, these terms are often used. Make practical decisions.

Answers

1. A is a concentrated solution of a non-electrolyte solute.
B is a dilute solution of a weak electrolyte solute.
C is a dilute solution of a strong electrolyte solute.
D is a dilute solution of a non-electrolyte solute.
E is a concentrated solution of a weak electrolyte solute.
F is a A concentrated solution of a strong electrolyte solute

2.
(a) These are nonsense questions. Words describing strength (such as strong, weak, stronger, weaker, …..) are relevant to properties of solutes in solution, not of the solutions.
(b) All of the following are valid statements about the solutions:

A is a more concentrated solution (of sucrose) than both B and C.
C is a more dilute (or, less concentrated) solution than both A and B.
B is a more dilute solution than A, but is more concentrated than C

3.
(a) All of the following are valid statements about the solutes:

HX is a stronger (weak) electrolyte than either of HY or HZ.
HZ is a weaker (weak) electrolyte than both HX and HY.
HY is a weaker electrolyte than HX, but a stronger electrolyte than HZ.

(b) The percentage of a weak electrolyte that ionises depends on the concentration of the solution. [The more dilute the solution, the more of the electrolyte ionises.] So the strengths of electrolytes can only be compared in solutions of the same concentration. (Although, in the example presented, the solutions would need to be of vastly different concentrations for the percentages ionised to be in a different order.]

4. Nonsense. Misuse of terms.

5. Solution A is more dilute … (that is less .. concentrated …) than solution B. Solution B is … more concentrated .. than solution C. The solute in solution A is a ... stronger .. electrolyte than the solute in solution B. In fact, because all of the HCl molecules ionize in solution, hydrochloric acid is a …. strong electrolyte …. . The solute in solution B is a …. weak electrolyte ... because only some of the molecules ... ionise … . in solution, but it is a … stronger electrolyte … than the solute in solution C. In solution C, none of the sucrose molecules ionise, so sucrose is called a … non-electrolyte .. .

6. There are many possible ways to represent the relationships among these categories of solution. One way is shown. The various solutions have been placed in their appropriate areas.

There is usually more than one way of diagrammatically representing relationships. No matter how the relational diagram is constructed, it should show:

Of all the aqueous solutions of substances, some are solutions of non-electrolytes, and some are solutions of electrolytes.
Of the aqueous solutions of electrolytes, some are solutions of strong electrolytes, and some are solutions of weak electrolytes.
Regardless of whether the solute is a strong electrolyte, a weak electrolyte, or a non-electrolyte, solutions may be either concentrated or dilute.

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