Concentration of Acids and Alkali
Solute, Solvent And Solution
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If a substance dissolves in a liquid, it is called a solute and the liquid is called the solvent. The resulting mixture is called a solution.
Solute + Solvent = Solution
Concentration Of A Solution
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The concentration of a solution tells you how much solute is dissolved in 1 unit volume of solution.
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The volume of a solution is measured in dm³ (litres). 1 dm³ = 1000 cm³.
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The amount of solute can be measured in grams or moles.
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2 units of concentration used in chemistry are g dm-3 and mol dm-3
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Concentration in g dm-3
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Concentration is the number of moles of solute per liter of solution.
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A concentration of 10 g dm-3 means there is 10 g of solute dissolved in1 dm3 of solution.
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Example
Calculate the concentration of the solution if 28g of NaOH is dissolve in 250cm3 of water.
Answer:
Mass of solute = 28g
Volume of solvent = 250cm³ = 0.25dm³
Concentration= 28g/0.25dm-3 = 112g dm-3
2. Concentration in mol dm-3 (Molarity)
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Molarity is probably the most commonly used unit of concentration. It is the number of moles of solute per liter of solution.
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A concentration of 2 mol dm-3 means there are 2 moles of solute dissolved in 1 dm3 of solution.
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Example
What is the molarity of a solution made when water is added to 0.2 mol of CaCl2 to make 100 cm³ of solution? [RAM: Ca = 40; Cl = 35.5]
Answer
Number of mole of solute = 0.2 mol
Volume of solvent = 100 cm³ = 0.1 dm³
Conversion of Certain Unit
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The chart above shows how to convert the units of concentration from g dm-3 to mol dm-3and vice versa.
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The molar mass of the solute is equal to the relative molecular mass of the solute.
Example 1:
The concentration of a Potassium chloride solution is 14.9 g dm-3. What is the molarity ( mol dm-3) of the solution? [ Relative Atomic Mass: Cl = 35.5; K = 39 ]
Answer:
Relative Formula Mass of Potassium Chloride (KCl)
= 39 + 35.5 = 74.5
Molar Mass of Potassium Chloride = 74.5 g/mol
Molarity of Potassium Chloride;
Example 2
A solution of barium hydrokxide have molarity 0.1 mol dm-3. What is the concentration of the solution in g dm-3? [Relative Atomic Mass: Ba = 137; O = 16; H = 1 ]
Answer:
Relative Formula Mass of barium hydrokxide, Ba(OH)2
= 137 + 2(16+1) = 171
Molar Mass of Potassium Chloride = 171 g/mol
Molarity And Number Of Moles
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Number of mole of solute in a solution can be calculated by using the following formula:
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where
n = number of mole of solute
M = molaruty of the solution
V = volume of the solution in cm3
Example:
How many moles of zinc sulphate is present in 200cm3 of 0.1 mol dm-3 zinc sulphate solution?
Answer:
Molarity, M = 0.1 mol dm-3
Voloume, V = 200cm3
Molarity And The PH Value Of Acid Or Alkali
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The table below shows the pH value and the molarity of a strong acid and a weak acid.
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The table below shows the pH value and the molarity of a strong alkali and a weak alkali.
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According to the tables shown above, we can conclude that the pH value of acid or alkali is affected by 2 factors:
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The Molarity
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Strong or weak acid(Alkali)
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Concentration Of Acid
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We have learned that the acidity of a solution is owing to the presence of hydrogen ions in a solution.
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We need to distinguish between the concentration of acid and the concentration of hydrogen ions of an acid.
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In most cases, the concentration of hydrogen ions has greater significant than the concentration of the acid.
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The concentration of hydrogen ion in acid depends on the
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molarity (concentration) of the acid
The higher the molarity of an acid, the higher the concentration (or molarity) of the hydrogen ions in the acid -
strength of the acid
Strong acid has higher concentration of hydrogen ions compare with weak acid of same concentration and same basicity -
basicity of the acid
The concentration of hydrogen ions of a diprotic acid is higher than the monoprotic acid and lower than the triprotic acid
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Example:
Calculate the number of mol of hydrogen ions that contain in 200cm3 of sulphuric acid 0.4 mol dm-3.
Answer:
Number of mole of sulphuric acid
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Sulphuric acid is a diprotic acid. Each molecule of sulphuric acid will produce 2 hydrogen ions when dissolve in water.
Number of mole of hydrogen ions
= 2 x Number of mole of sulphuric acid
= 2 x 0.08 = 0.16 mol
Dilution
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Dilution is a process of adding water to the standard solution lowered the concentration of the solution.
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In dilution of solution, we should take note that mole of solute before dilution is equal to the mole of solute after dilution.
Number of mol of solute before dilution = Number of mole of solute after dilution
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M1 = Molarity before dilution
M2 = Molarity after dilution
V1 = Volume before dilution
V2 = Volume after dilution
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Example 1
100cm3 of 0.5 mol dm-3 sodium chloride solution is diluted with distilled water to produce 250 cm3 of solution. Calculate the concentration (in mol dm-3) of the sodium chloride solution after the dilution.
Answer:
M1 = 0.5 mol dm-3
M2 = ?
V1 = 100cm3
V2 = 250 cm3
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Example 2
Find the volume of 2 mol/dm3 nitric acid that needs to be diluted with distill water to produce 500cm3 of 0.05 mol/dm3 nitric acid.
Answer:
M1 = 2 mol dm-3
M2 = 0.05 mol/dm3
V1 = ？
V2 = 500 cm3
Preparing A Standard Solution
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A standard solution is a solution in which its concentration is known.
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The steps taken in preparing a standard solution are:
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Determine the volume and concentration that you want to prepare.
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Calculate the mass of solute needed to give the required volume and concentration.
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Weigh the solute
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Dissolve the solute completely dissolved in distilled water and then transfer it to a volumetric flask partially filled with distilled water.
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Add distilled water to the calibration mark of the volumetric flask.
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Invert the flask and shake it to make sure thorough mixing.
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