Analysing concentration of acids and alkalis
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Malaysia SPM Form 4 Chemistry, Chapter 7: Acid and Base.
Contents |
[edit] Introduction
| Solute | Solid which dissolves |
| Solvent | Liquid which does the dissolving |
| Solution | Solute + solvent |
| Saturated solution | A solution which will dissolve no more solute. |
| Soluble | A substance that will dissolve in a solvent. |
| Insoluble | A substance that will not dissolve in a solvent |
- 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.
- The volume of a solution is measured in dm3 (litres).
- The concentration of a solution tells you how much solute is dissolved in 1 dm3 of solution.
The amount of solute can be measured in grams or moles.
[edit] Concentration
[edit] Concentration (g dm-3)
Concentration is the number of moles of solute per liter of solution. A concentration of 10 g dm-3 means there is 10 g of solute dissolved in1 dm3 of solution.
| Example 1 |
Calculate the concentration of the solution if 28g of NaOH is dissolve in 250cm3 of water. |
| 'Answer' Mass of the solute, NaOH = 28g Volume of the solvent = 250cm3 = 0.250dm3 [The unit of concentration is always in g/dm3</p> |
[edit] Molarity (M)
Molarity is probably the most commonly used unit of concentration. It is the number of moles of solute per liter of solution. A concentration of 2 mol dm-3 means there are 2 moles of solute dissolved in 1 dm3 of solution.
| Example 2 |
| What is the molarity of a solution made when water is added to 11 g CaCl2 to make 100 cm3 of solution? [RAM: Ca = 40; Cl = 35.5] |
| Example 3 |
| What is the molarity of a solution prepares by dissolving 117g NaCl in enough water to make 500cm3 of solution?
[Relative Atomic Mass: Na = 23; Cl= 35.5 ] |
[edit] Conversion of Concentration Unit
| Example 4 |
| 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 ] |
| Example 5 |
| 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 ] |
| Example 6 |
| Molarity of acid X is 0.4 mol dm-3. Find the relative molecular mass of acid X if its concentration is 14.6g dm-3. |
[edit] 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.
| Molarity of Acis | pH value | |
| Hydrochloric acid | Ethanoic Acid | |
| 0.1 mol dm-3 | 1.0 | 2.9 |
| 0.01 mol dm-3 | 2.0 | 3.4 |
| 0.001 mol dm-3 | 3.0 | 3.9 |
| 0.0001 mol dm-3 | 4.0 | 4.4 |
| 0.00001 mol dm-3 | 5.0 | 4.9 |
The table below shows the pH value and the molarity of a strong alkali and a weak alkali.
| Molarity of alkali | pH value | |
| Sodium hydroxide solution | Ammonia aqueus | |
| 0.1 mol dm-3 | 13.0 | 11.1 |
| 0.01 mol dm-3 | 12.0 | 10.6 |
| 0.001 mol dm-3 | 11.0 | 10.1 |
| 0.0001 mol dm-3 | 10.0 | 9.6 |
| 0.00001 mol dm-3 | 9.0 | 9.1 |
- According to the tables shown above, we can conclude that the pH value of acid or alkali is affected by 2 factors:
- The Molarity
- Strong or weak acid(Alkali)
| Example 7 |
Which of the following solution has higest pH value?
|
[edit] Molarity and Mole
- Number of mole of solute in a solution is given by the equation
M = molarity V = volume of solution in cm3
[edit] Mole of Ion
[edit] Dilution
- 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
[edit] Concentration of Acid
- Concentration of hydrogen ion in acid depends on
- Molarity(concentration) of the acid
- Strong acid or weak acid
- Basicity of the acid
| Example 8 |
| Calculate the number of mol of hydrogen ions that contain in 200cm3 of sulphuric acid 0.4 mol dm-3. |
[edit] Preparing A standard Solution
- A standard solution is a solution whose concentration was known.
- Usually, a standard solution is prepared by volumetric flask.
- In the process, you need to calculate the mass of the solute and the volume of the solvent.
| Example 9 |
| You are asked to prepare 250cm3 of the solution of sodium hydroxide, 0.5 moldm-3. Find out the mass of sodium hydroxide that you need to dissolve into the solvent.
[ Relative atomic mass: Na=23, O = 16; H=1] Answer The mol of NaOH =
=
= |
| Example 10 |
| What is the volume of water that need to be added into 14.9g of potassium chloride to obtain a solution of potassium chloride, 0.4 moldm-3.
[ Relative atomic mass: K=39, Cl = 35.5] |
| Example 11 |
| 4g of sodium hydroxide is added into 250 cm3 of water. What is the molarity of the solution yield?
[ Relative atomic mass: Na=23, O = 16; H=1] |
[edit] Stoichiometry of Reaction
- The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry, so be prepared for lots of calculator use.
- The Steps Involved in Solving Mass-Mass Stoichiometry Problems
- Make sure the chemical equation is correctly balanced.
- Using the molar mass of the given substance, convert the mass given in the problem to moles.
- Construct a molar proportion (two molar ratios set equal to each other) following the guidelines set out in other files. Use it to convert to moles of the unknown.
- Using the molar mass of the unkown substance, convert the moles just calculated to mass.
