Problem 6 - Chemistry in 30 days

Question

The amount of oxalic acid (mol. wt. 63 ) required to prepare \(500 \mathrm{ml}\) of is \(0.10 \mathrm{~N}\) solution is (a) \(0.315 \mathrm{~g}\) (b) \(3.150 \mathrm{~g}\) (c) \(6.300 \mathrm{~g}\) (d) \(63.00 \mathrm{~g}\)

Step-by-Step Solution

Answer

None of the options match exactly; calculation gives 1.575 g.

Step 1: Calculate Equivalent Weight of Oxalic Acid

Oxalic acid (H2C2O4) has a molecular weight of 63. The equivalent weight is calculated by dividing the molecular weight by the number of equivalence factor involved in its neutralization. Oxalic acid has two hydrogens that can be ionized, so the equivalence factor is 2. Therefore, its equivalent weight is \( \frac{63}{2} = 31.5 \).

Step 2: Determine Normality Formula

Normality (\( N \)) is calculated using the formula \( N = \frac{\text{Equivalent weight} \times \text{Volume (L)}}{\text{Mass (g)}} \). We need to rearrange this to find the mass: \( \text{Mass (g)} = \frac{N \times \text{Equivalent weight} \times \text{Volume (L)}}{1} \).

Step 3: Convert Volume to Liters

The volume given is in milliliters. Convert this to liters by dividing by 1000: \( 500 \, \text{ml} = 0.5 \, L \).

Step 4: Calculate the Required Mass of Oxalic Acid

Substitute the values into the formula from Step 2. Here, \( N = 0.10 \), \( \text{Equivalent weight} = 31.5 \), and \( \text{Volume} = 0.5 \text{ L} \). \[ \text{Mass (g)} = 0.10 \times 31.5 \times 0.5 = 1.575 \text{ g} \].

Step 5: Correct Answer Selection

Since the calculated mass required is \( 1.575 \text{ g} \), compare it with the options provided. None of the options exactly match \( 1.575 \text{ g} \), but if considering whole numbers and possible approximation in an exam setting, the closest logical weight would likely be calculated differently if matching one of them.

Key Concepts

Equivalent Weight

Understanding equivalent weight is essential in chemistry, especially when dealing with acids like oxalic acid. The equivalent weight of a substance is calculated by dividing its molecular weight by the number of equivalents of the substance that react. For oxalic acid, with a molecular weight of 63, this calculation is slightly simpler since oxalic acid has two reactive hydrogen ions.
This means, in neutralization reactions, each molecule can donate two protons. Therefore, the equivalence factor is 2. Hence, the equivalent weight of oxalic acid is computed as:
\[ \text{Equivalent Weight} = \frac{\text{Molecular Weight}}{\text{Equivalence Factor}} = \frac{63}{2} = 31.5 \] Understanding this concept helps in later calculations for making solutions and determining their concentration.

Normality Calculation

Normality is a concentration term that emphasizes the reactive capacity of a compound in solution. It is particularly useful when dealing with acids and bases. To find the normality of a solution, we need to know the equivalent weight calculated earlier and use it in combination with the volume and mass of the substance.
The normality formula derives from the relationship:

• \[ N = \frac{\text{Equivalent Weight} \times \text{Volume (L)}}{\text{Mass (g)}} \]

If we rearrange this equation to solve for mass — a common requirement when preparing solutions — it becomes:

• \[ \text{Mass (g)} = N \times \text{Equivalent Weight} \times \text{Volume (L)} \]

This formula indicates how much of the chemical we need to obtain a solution of a certain normality. In this exercise, knowing the volume in liters and the equivalent weight enables the calculation of the mass needed.

Solution Preparation

Preparing a solution with a specific concentration involves careful calculation and accurate measurement. Here's a step-by-step overview of how to prepare a desired concentration:

• First, convert the solution volume from milliliters to liters, because normality calculations require volume in liters. For example, 500 ml is converted to 0.5 L.


• With the formula \( \text{Mass (g)} = N \times \text{Equivalent Weight} \times \text{Volume (L)} \), plug in the values: Normality \( (N) = 0.10 \), Equivalent weight = 31.5, and Volume = 0.5 L.


• The calculation results in a mass of 1.575 grams. This would be the exact mass of oxalic acid needed to make 500 ml of a 0.10 N solution.

Solution preparation doesn't only involve calculating the correct mass but also accurately weighing this mass for the solution. It’s crucial to ensure precision to achieve the correct concentration. This foundational knowledge will assist anyone learning about solution preparation.