Acid-Base Equivalents: Videos & Practice Problems

In the study of acids and bases, the concept of equivalents is crucial for understanding their reactivity and neutralization. An equivalent of an acid is defined as the amount of acid that can donate one mole of hydrogen ions (H⁺), while an equivalent of a base is the amount that can provide one mole of hydroxide ions (OH^-). This relationship allows chemists to quantify the strength and capacity of acids and bases in reactions.

For instance, consider hydrobromic acid (HBr). One mole of HBr yields one mole of H⁺ ions, thus it is equivalent to one equivalent of acid. In contrast, calcium hydroxide (Ca(OH)₂) contains two hydroxide ions per formula unit. Therefore, one mole of calcium hydroxide corresponds to two equivalents, as it can donate two moles of OH^- ions.

Understanding the concept of equivalents becomes particularly important when dealing with reactions involving different amounts of acids and bases. The varying number of equivalents can influence the outcome of neutralization reactions, where the stoichiometry of the reactants determines the final products. This foundational knowledge sets the stage for deeper exploration into acid-base chemistry and the calculations involved in titrations and other analytical methods.

To determine the number of equivalents of an acid or base, the formula used is:

Equivalents = n × moles

Here, n represents the number of moles of hydrogen ions (H⁺) for acids or hydroxide ions (OH^-) for bases. This relationship is crucial for understanding acid-base chemistry.

Additionally, the concept of milliequivalents (mEq) is commonly used to express equivalence in a more manageable unit. One equivalent is equivalent to 1,000 milliequivalents:

1 Equivalent = 1,000 mEq

For acids, the equivalence can be calculated as:

Acid Equivalence = n × moles of acid

For bases, the equivalence is given by:

Base Equivalence = n × moles of base

Understanding these calculations is essential for accurately measuring and comparing the reactivity of acids and bases in various chemical reactions.

To calculate the number of equivalents in a substance, it is essential to understand the relationship between moles and the number of reactive species, such as hydrogen ions (H⁺) for acids and hydroxide ions (OH^-) for bases.

For phosphoric acid (H₃PO₄), which is an acid, the number of equivalents can be determined using the formula:

Equivalents = n × moles

Here, n represents the number of H⁺ ions that the acid can donate. Phosphoric acid has three H⁺ ions, so:

n = 3

If we have 1 mole of phosphoric acid, the calculation becomes:

Equivalents = 3 × 1 mole = 3 equivalents

Next, consider the base, rubidium hydroxide (RbOH). For bases, the number of equivalents is calculated similarly, where n is the number of OH^- ions. RbOH contains one OH^- ion, so:

n = 1

To find the number of equivalents for RbOH, we first need to convert grams to moles. The molar mass of RbOH is calculated by adding the atomic masses of rubidium (Rb), oxygen (O), and hydrogen (H). The molar mass is approximately 102.476 grams per mole. If we have a certain mass of RbOH, we can convert it to moles:

Moles = mass (g) / molar mass (g/mol)

Assuming we have 2.66 grams of RbOH, the calculation would be:

Moles = 2.66 g / 102.476 g/mol ≈ 0.026 moles

Now, substituting this value into the equivalents formula for the base:

Equivalents = 1 × 0.026 moles = 0.026 equivalents

In summary, for phosphoric acid, there are 3 equivalents per mole, while for rubidium hydroxide, there are 0.026 equivalents based on the calculated moles. This method allows for the identification and calculation of equivalents for both acids and bases effectively.

The concept of equivalent weight is crucial in understanding acid-base chemistry. It refers to the mass in grams of one equivalent of an acid or base. To calculate the equivalent weight, you can use the formula:

Equivalent Weight = \(\frac{\text{Molar Mass}}{n}\)

In this equation, n represents the number of reactive units in the acid or base. For acids, n corresponds to the number of hydrogen ions (\(H^+\)) that can be donated, while for bases, it is the number of hydroxide ions (\(OH^-\)) that can be accepted. This relationship helps in stoichiometric calculations, allowing chemists to determine how much of a substance is needed to react with a given amount of another substance in a chemical reaction.

To calculate the equivalent weight of sulfuric acid (H₂SO₄), we use the formula for equivalent weight, which is defined as the molar mass of the acid divided by the number of hydrogen ions (H⁺) it can donate. In the case of sulfuric acid, it can donate 2 H⁺ ions, so n equals 2.

The molar mass of sulfuric acid is determined by summing the atomic masses of its constituent elements: 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms. Referring to the periodic table, the atomic masses are approximately:

• Hydrogen (H): 1.008 g/mol
• Sulfur (S): 32.06 g/mol
• Oxygen (O): 16.00 g/mol

Calculating the molar mass:

2(1.008) + 32.06 + 4(16.00) = 2.016 + 32.06 + 64.00 = 98.076 g/mol

Thus, the molar mass of sulfuric acid is approximately 98.086 g/mol. To find the equivalent weight, we divide this value by the number of H⁺ ions:

Equivalent weight = \(\frac{98.086 \, \text{g/mol}}{2} = 49.043 \, \text{g/mol}\)

Therefore, the equivalent weight of sulfuric acid is 49.043 grams per mole.

Normality is a crucial concept in chemistry that quantifies the concentration of acids or bases in aqueous solutions. It is defined as the number of equivalents of a solute per liter of solution. This is distinct from molarity, which measures the concentration in terms of moles of solute per liter of solution. The relationship between normality and molarity can be expressed through the formula:

$$ N = n \times M $$

In this equation, N represents normality, n denotes the number of equivalents, and M is the molarity. For acids, n corresponds to the number of hydrogen ions (H⁺) that can be donated, while for bases, it refers to the number of hydroxide ions (OH^-) that can be accepted. Understanding this relationship allows for the effective calculation of the concentration of acidic or basic solutions, facilitating various applications in chemical reactions and titrations.

To calculate the normality of solutions, it's essential to understand the relationship between normality and molarity. Normality (N) is defined as the number of equivalents of solute per liter of solution. For bases, the number of equivalents corresponds to the number of hydroxide ions (OH^-) produced per formula unit of the base.

For the first solution (a), we have sodium hydroxide (NaOH) with a concentration of \(4.6 \times 10^{-2}\) M. Since sodium hydroxide dissociates to produce one hydroxide ion, the normality is calculated as follows:

Normality = \(N \times \text{Molarity}\)

Here, \(N = 1\) (for one OH^- ion), so:

Normality = \(1 \times 4.6 \times 10^{-2} = 4.6 \times 10^{-2}\) N.

For the second solution (b), we have 0.35 grams of phosphoric acid (H₃PO₄) in 1 liter of solution. To find the normality, we first need to determine the number of equivalents. Phosphoric acid can donate three protons (H⁺ ions), so:

Equivalents = \(n \times \text{moles}\)

Where \(n = 3\) (for three H⁺ ions). First, we calculate the moles of phosphoric acid:

To find the moles, we use the molar mass of H₃PO₄, which is approximately 97.994 g/mol:

Moles = \(\frac{0.35 \text{ g}}{97.994 \text{ g/mol}} \approx 0.0035716 \text{ moles}\)

Now, we can calculate the equivalents:

Equivalents = \(3 \times 0.0035716 \approx 0.0107148\) equivalents.

Finally, we can find the normality:

Normality = \(\frac{\text{Equivalents}}{\text{Liters of solution}} = \frac{0.0107148}{1} = 0.0107148\) N.

Considering significant figures, since 0.35 has two significant figures, the normality is rounded to:

Normality = 0.011 N.

In summary, the normalities for the solutions are:

Solution (a): \(4.6 \times 10^{-2}\) N

Solution (b): 0.011 N