Question

How many grams of sodium hydroxide present in 250 ml. of 0.5 M NaOH solution?

• A. 7.32 g
• B. 0.005 g
• C. 5g
• D. 0.5 g

Answer 1

Answer:

C. 5g

Understanding Molarity and Solution Calculations

• What is Molarity (M)?

  • Molarity is a measure of the concentration of a solute in a solution.
  • It is defined as the number of moles of solute per liter of solution.
  • Its unit is moles/liter, often denoted as M.
  • Formula: Molarity (M) = (Moles of solute) / (Volume of solution in Liters)

• Calculating Moles from Molarity and Volume

  • Rearranging the molarity formula, we get: Moles of solute = Molarity (M) × Volume of solution (L).
  • It is crucial to convert the volume from milliliters (mL) to liters (L) before calculation. 1 Liter = 1000 mL.

• Determining Molar Mass

  • The molar mass of a compound is the mass of one mole of that compound, expressed in grams per mole (g/mol).
  • For Sodium Hydroxide (NaOH):
    • Atomic mass of Sodium (Na) = 23 g/mol
    • Atomic mass of Oxygen (O) = 16 g/mol
    • Atomic mass of Hydrogen (H) = 1 g/mol
    • Therefore, Molar Mass of NaOH = 23 + 16 + 1 = 40 g/mol.

• Calculating Mass from Moles and Molar Mass

  • Once the moles of solute are known, the mass can be calculated using the formula: Mass (grams) = Moles (mol) × Molar Mass (g/mol).

Step-by-Step Calculation for the Given Problem

1. Convert Volume to Liters:
   • Given volume = 250 mL
   • Volume in Liters = 250 mL / 1000 mL/L = 0.25 L
2. Calculate Moles of NaOH:
   • Given Molarity (M) = 0.5 M
   • Volume (L) = 0.25 L
   • Moles of NaOH = Molarity × Volume = 0.5 mol/L × 0.25 L = 0.125 moles
3. Calculate Mass of NaOH:
   • Moles of NaOH = 0.125 mol
   • Molar Mass of NaOH = 40 g/mol
   • Mass of NaOH = Moles × Molar Mass = 0.125 mol × 40 g/mol = 5 grams

Important Facts about Sodium Hydroxide (NaOH) for Competitive Exams

• Common Names:

  • NaOH is commonly known as Caustic Soda or Lye.

• Nature and Properties:

  • It is a strong base, meaning it completely dissociates in water to produce hydroxide ions (OH-).
  • It is a white, crystalline solid at room temperature.
  • NaOH is highly deliquescent, meaning it readily absorbs moisture from the air and dissolves in it.
  • It is highly soluble in water and generates significant heat upon dissolution (exothermic reaction).

• Major Uses:

  • Widely used in the manufacturing of soap and detergents.
  • Used in the production of pulp and paper.
  • Essential in the textile industry for mercerizing cotton.
  • Employed in water treatment, petroleum refining, and food processing.
  • Used in drain cleaners due to its ability to react with fats and grease.

• Safety Precautions:

  • NaOH is highly corrosive and can cause severe chemical burns to skin and eyes.
  • Appropriate personal protective equipment (PPE) like gloves and safety goggles should always be used when handling it.