Question

\(4 \mathrm{~g}\) caustic soda is dissolved in 100 cc of solution. The normality of solution is: (a) 1 (b) \(0.8\) (c) \(0.6\) (d) \(0.10\)

Short answer

The normality of the solution is 1 (option a).

Step-by-step solution

Determine the Equivalent Weight

To find the normality of a solution, we first need the equivalent weight of the solute. Caustic soda is NaOH, which has a molecular weight of 40 g/mol. Since NaOH is a monoprotic base (it donates one OH⁻ ion), its equivalent weight is the same as its molecular weight, which is 40 g/eq.

Calculate the Number of Equivalents

Next, calculate the number of equivalents using the formula: \( ext{Number of equivalents} = \frac{ ext{Mass of solute (g)}}{ ext{Equivalent weight (g/eq)}}\)Substitute the given values: \( ext{Number of equivalents} = \frac{4 ext{ g}}{40 ext{ g/eq}} = 0.1 ext{ eq}\)

Convert Volume from cc to Liters

The volume of the solution is given in cubic centimeters (cc). To find normality, we need the volume in liters (L). Since 1000 cc = 1 L, we convert 100 cc as follows: \(100 ext{ cc} = 0.1 ext{ L}\)

Calculate the Normality

Normality (N) is given by the formula:\(N = \frac{ ext{Number of equivalents}}{ ext{Volume of solution in liters}}\)Now substitute the values calculated:\(N = \frac{0.1 ext{ eq}}{0.1 ext{ L}} = 1\)

Key concepts

Equivalent Weight

The concept of equivalent weight is essential in determining the normality of a solution. Equivalent weight refers to the amount of a substance that reacts with, or is equivalent to, one mole of hydrogen ions (H⁺) or hydroxide ions (OH⁻). It is calculated by dividing the molecular weight of a compound by the number of electrons it can donate or accept in a reaction.
For instance, in the case of sodium hydroxide (NaOH), a common base also known as caustic soda, the molecular weight is 40 g/mol. Because NaOH can donate one hydroxide ion (OH⁻), it's considered a monoprotic base. Therefore, its equivalent weight is the same as its molecular weight: 40 g/eq.
Understanding equivalent weight helps in accurately calculating the number of equivalents, particularly in acid-base reactions. It simplifies calculations and allows chemists to easily compare different substances.

Monoprotic Base

A monoprotic base is a substance that can donate one hydroxide ion (OH⁻) per molecule when it dissolves in a solution. This is a straightforward concept but important to grasp for understanding chemical reactions involving bases.
Sodium hydroxide (NaOH) is a classic example of a monoprotic base. When NaOH is dissolved in water, it dissociates completely into sodium ions (Na⁺) and hydroxide ions (OH⁻).

• This one-to-one donation makes NaOH a monoprotic base, which simplifies the calculation of its equivalent weight and normality.
• Knowing whether a base is monoprotic can significantly affect how calculations are approached, ensuring accurate measurements and preparations in experimental settings.

Number of Equivalents

The number of equivalents is a key part of calculating the normality of a solution. It measures how much of a substance can participate in a chemical reaction, often tied to the concept of valency.
Calculating the number of equivalents involves using the formula:
\[\text{Number of equivalents} = \frac{\text{Mass of solute (g)}}{\text{Equivalent weight (g/eq)}}\]
For our example, with 4 grams of NaOH and an equivalent weight of 40 g/eq, the number of equivalents is:
\[\frac{4 \text{ g}}{40 \text{ g/eq}} = 0.1 \text{ eq}\]

• This means, in the solution, 0.1 equivalents of NaOH are available to react, which is crucial for determining the solution's normality.
• Knowing how to calculate equivalents ensures the right concentration and reactivity is achieved for any given reaction.

Volume Conversion

Volume conversion is a common task in chemistry, especially important for concentration calculations. To calculate normality, volumes must be expressed in liters (L) rather than cubic centimeters (cc) because standard concentration formulas use liters.
In our example, the solution's volume is given as 100 cc. Since 1000 cc is equal to 1 L, converting 100 cc to liters can be done by dividing by 1000:
\[100 \text{ cc} = 0.1 \text{ L}\]
Converting volume units accurately is essential for calculating normality or molarity correctly:

• This small step ensures that the number of equivalents can be correctly divided by the volume, providing the true normality of the solution.
• Being consistent in units prevents errors and supports the integrity of experimental results.