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Questions and Answers
Which of the following characteristics does NOT describe a primary standard substance?
Which of the following characteristics does NOT describe a primary standard substance?
- It can be dried at 105-110°C without decomposition.
- It must react quantitatively with other substances.
- It is a solution of approximately known concentration. (correct)
- It must be easily obtained in a high grade of purity.
What process is used to determine the exact concentration of a secondary standard solution?
What process is used to determine the exact concentration of a secondary standard solution?
- Measurement adjustment
- Standardization (correct)
- Concentration validation
- Calibration
Which of the following statements about standard solutions is accurate?
Which of the following statements about standard solutions is accurate?
- Primary standard solutions cannot be prepared by weighing.
- Standard solutions are classified only as primary standards.
- Primary standards remain constant in concentration over long periods. (correct)
- Secondary standard solutions have a fixed concentration.
Which of the following substances would be classified as a primary standard?
Which of the following substances would be classified as a primary standard?
What must a primary standard substance be able to do in chemical reactions?
What must a primary standard substance be able to do in chemical reactions?
What does 0.5 M NaOH represent in terms of its molarity?
What does 0.5 M NaOH represent in terms of its molarity?
Which of the following correctly defines normality (N) in a solution?
Which of the following correctly defines normality (N) in a solution?
What does an equivalent weight of a substance signify?
What does an equivalent weight of a substance signify?
If the molecular weight of NaOH is 40 g/mol, how much would 2 moles of NaOH weigh?
If the molecular weight of NaOH is 40 g/mol, how much would 2 moles of NaOH weigh?
What is the primary factor differentiating molarity from normality?
What is the primary factor differentiating molarity from normality?
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Study Notes
Acid-Base Titrations: Standard Solutions
- Standard solutions are solutions with a known concentration and composition.
- Primary Standard solutions have a precisely known concentration and are stable over time.
- They are prepared by dissolving a precisely weighed amount of a primary standard substance in a volumetric flask.
- Requirements for a primary standard substance:
- High purity and known composition.
- Stable and resistant to reactions with air.
- Not deliquescent or hygroscopic, and nonvolatile.
- Can be dried at 105-110°C without decomposition.
- High molecular weight to minimise weighing errors.
- Reacts quantitatively with other substances.
- Examples of primary standard substances:
- Acids: Potassium acid phthalate, Oxalic acid, Benzoic acid.
- Bases: Sodium carbonate, Borax (sodium borate).
- Secondary Standard solutions have an approximately known concentration.
- Their exact concentration is determined by a process called standardization.
- Standardization involves:
- Titrating the secondary standard against a primary standard solution or a previously standardized secondary standard.
- Examples of secondary standards:
- HCl and NaOH.
Methods of Expressing Concentration
- The most common methods for expressing the concentration of solutions are:
- Molarity (M) - The number of moles of solute per liter of solution.
- Normality (N) – The number of equivalent weights of solute per liter of solution.
Molarity
- Equation:
M = Number of moles of solute / Volume in L of solution - Example: 0.5 M NaOH means 0.5 moles of NaOH in 1 L of the solution.
Mole (mol)
- For molecules: The mole is the gram-molecular weight (molecular weight expressed in grams).
- For atoms: The mole is the gram-atomic weight (atomic weight expressed in grams).
- Example:
- The molecular weight of NaOH is 40, therefore 1 mole of NaOH weighs 40 grams.
- The atomic weight of Na is 23, therefore 1 mole of Na weighs 23 grams.
Normality
- Equation:
N = Number of equivalent weights of solute / Volume in L of solution - Example: 2 N HCl means 2 equivalent weights of HCl per 1 L of the solution.
Equivalent Weight
- It is the weight of a substance that is equivalent in reactivity to 1 mole of hydrogen.
- The exact definition and calculation of equivalent weight vary depending on the type of reaction taking place.
Acid-Base Reactions
- We will focus on acid-base reactions in this section.
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