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Concentrations

Solute, Solvent And Solution

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

  1. The concentration of a solution tells you how much solute is dissolved in 1 unit volume of solution.

  2. The volume of a solution is measured in dm³ (litres).  1 dm³ = 1000 cm³.

  3. The amount of solute can be measured in grams or moles.

  4. 2 units of concentration used in chemistry are g dm-3 and mol dm-3

Concentration in g dm-3

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  1. Concentration is the number of moles of solute per liter of solution.

  2. A concentration of 10 g dm-3 means there is 10 g of solute dissolved in1 dm3 of solution.

Concentration = Mass of solute(g) / Volume of solution(dm3)

Concentration in mol dm-3 (Molarity)

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  1. Molarity is probably the most commonly used unit of concentration. It is the number of moles of solute per liter of solution.

  2. A concentration of 2 mol dm-3 means there are 2 moles of solute dissolved in 1 dm3 of solution.

Molarity = Mole of solute(mol) / Volume of solution(dm3)

Conversion Of Concentration Unit

  1. The chart above shows how to convert the units of concentration from g dm-3 to mol dm-3and vice versa.

  2. The molar mass of the solute is equal to the relative molecular mass of the solute.

Molarity And Number Of Moles

Number of mole of solute in a solution can be calculated by using the following formula

n = MV / 1000

where
n = number of mole of solute
M = molaruty of the solution
V = volume of the solution in cm3

Molarity And The PH Value Of Acid Or Alkali

The table below shows the pH value and the molarity of a strong acid and a weak acid.

The table below shows the pH value and the molarity of a strong alkali and a weak alkali.

  1. ​According to the tables shown above, we can conclude that the pH value of acid or alkali is affected by 2 factors:

    1. The Molarity

    2. Strong or weak acid(Alkali)

Concentration Of Acid

  1. We have learned that the acidity of a solution is owing to the presence of hydrogen ions in a solution.

  2. We need to distinguish between the concentration of acid and the concentration of hydrogen ions of an acid.

  3. In most cases, the concentration of hydrogen ions has greater significant than the concentration of the acid. 

  4. The concentration of hydrogen ion in acid depends on the

    1. 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

    2. strength of the acid
      Strong acid has higher concentration of hydrogen ions compare with weak acid of same concentration and same basicity

    3. 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

Dilution

  1. Dilution is a process of adding water to the standard solution lowered the concentration of the solution.

  2. In dilution of solution, we should take note that mole of solute before dilution is equal to the mole of solute after dilution.

M1 = Molarity before dilution
M2 = Molarity after dilution
V1 = Volume before dilution
V2 = Volume after dilution

Preparing A Standard Solution

  1. A standard solution is a solution in which its concentration is known.

  2. The steps taken in preparing a standard solution are:

    1. Determine the voloume and concentration that you want to prepare.

    2. Calculate the mass of solute needed to give the required volume and concentration.

    3. Weigh the solute

    4. Dissolve the solute completely dissolved in distilled water and then transfer it to a volumetric flask partially filled with distilled water.

    5. Add distilled water to the calibration mark of the volumetric flask.

    6. Invert the flask and shake it to make sure thorough mixing.

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