Q.Calculate the molarity of NaOH in the solution prepared by dissolving its 4 g in enough water to form 250 mL of the solution.

The key idea is Molarity Calculation.

First, determine the molar mass of NaOH.

Molar mass of NaOH $= (\text{Atomic mass of Na}) + (\text{Atomic mass of O}) + (\text{Atomic mass of H})$

Molar mass of NaOH $= (23.0 + 16.0 + 1.0) \, \text{g/mol} = 40.0 \, \text{g/mol}$.

Next, calculate the number of moles of NaOH dissolved.

Moles of NaOH $= \frac{\text{Mass of NaOH}}{\text{Molar mass of NaOH}} = \frac{4 \, \text{g}}{40 \, \text{g/mol}} = 0.1 \, \text{mol}$.

Convert the volume of the solution from milliliters to liters.

Volume of solution $= 250 \, \text{mL} = \frac{250}{1000} \, \text{L} = 0.25 \, \text{L}$. …

Molarity is a fundamental concept in chemistry that quantifies the concentration of a solute in a solution. It tells us how many moles of a substance are dissolved in one liter of the solution. Understanding molarity is crucial for many chemical calculations, especially in stoichiometry, titrations, and reaction kinetics.

The core idea is to relate the amount of solute (in moles) to the volume of the solution (in liters). We are given the mass of NaOH and the total volume of the solution. Our task is to convert the given mass into moles and the given volume into liters, then apply the definition of molarity.

Here's how we calculate the molarity step-by-step:

1. Identify the solute and its given mass.

   The solute is Sodium Hydroxide (NaOH), and its mass is given as $4 \text{ g}$.

2. Determine the molar mass of the solute (NaOH).

   The molar mass is the mass of one mole of a substance. We need the atomic masses of its constituent elements:

   • Sodium (Na): $23 \text{ g/mol}$
   • Oxygen (O): $16 \text{ g/mol}$
   • Hydrogen (H): $1 \text{ g/mol}$

   Adding these together gives the molar mass of NaOH:

3. Calculate the number of moles of NaOH. Now that we have the mass of NaOH and its molar mass, we can find the number of moles using the formula:

Substituting the values:

4. Convert the volume of the solution to liters. Molarity is defined for volume in liters. The given volume is $250 \text{ mL}$. We know that $1 \text{ L} = 1000 \text{ mL}$. …

Method: Molarity Formula Method

Molarity ($M$) is defined as the number of moles of solute per litre of solution.

Steps

Step 1: Write the molarity formula

Step 2: Calculate moles of NaOH

• Molar mass of NaOH = $23 + 16 + 1 = 40 \text{ g/mol}$
• Given mass = $4 \text{ g}$
• Moles of NaOH = $\frac{\text{given mass}}{\text{molar mass}} = \frac{4}{40} = 0.1 \text{ mol}$

Step 3: Convert volume to litres

• Given volume = $250 \text{ mL}$
• Volume in litres = $\frac{250}{1000} = 0.25 \text{ L}$

Step 4: Substitute into formula …

Here are the most common mistakes students make when solving this exact molarity problem, along with how to avoid each — explained concept-first.

1. Confusing Molarity with Molality

Mistake: Using mass of solvent (water) instead of volume of solution.

• Students often write: $\text{Molarity} = \frac{\text{moles of solute}}{\text{mass of solvent in kg}}$ This is molality, not molarity.

Why it happens: The terms sound similar, and both start with "mo-".

How to avoid:

• Memorise the definition: $\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{volume of solution in litres}}$
• The denominator is total volume of solution, not solvent.

2. Forgetting to Convert Volume to Litres

Mistake: Plugging in 250 mL directly without converting to litres.

• Wrong: $\text{Molarity} = \frac{0.1 \text{ mol}}{250} = 0.0004 \text{ M}$ (This gives a tiny, incorrect value.)

Why it happens: The formula says "litres", but the problem gives mL.

How to avoid:

• Always convert mL → L by dividing by 1000. $250 \text{ mL} = 0.250 \text{ L}$
• Write the conversion step explicitly: $\text{Volume in L} = \frac{250}{1000} = 0.25 \text{ L}$

3. Using Wrong Molar Mass of NaOH

Mistake: Using atomic masses incorrectly — e.g., Na = 23, O = 16, H = 1, but adding them wrong or using rounded values inconsistently.

• Common error: $23 + 16 + 1 = 40 \text{ g/mol}$ (correct) But some use 39.997 or 40.0 and then round mid-calculation.

Why it happens: Rushing or not checking the periodic table.

How to avoid:

• Write the sum clearly: $\text{Molar mass of NaOH} = 23 + 16 + 1 = 40 \text{ g/mol}$
• Use exact values as given in your exam (usually 40 g/mol for NaOH).
• Double-check addition — it’s a simple but common slip.

4. Calculating Moles Incorrectly

Mistake: Using mass in grams but dividing by molar mass in kg/mol, or vice versa.

• Wrong: $\text{Moles} = \frac{4 \text{ g}}{0.040 \text{ kg/mol}} = 100 \text{ mol}$ (huge error)

Why it happens: Mixing units (g vs kg).

How to avoid:

• Always use grams for mass and g/mol for molar mass. …