Acid-Base Equivalents: Videos & Practice Problems

Equivalents quantify the reactive capacity of acids and bases by measuring moles of H⁺ or OH⁻ ions contributed. Equivalent weight is calculated as molar mass divided by the number of ionizable H⁺ or OH⁻ ions ($\mathrm{Equivalent}\mathrm{weight}=\frac{\mathrm{Molar}}{\mathrm{<span\; class="iROOqd">mass</span>}}$). Normality (N) expresses concentration as equivalents per liter, related to molarity (M) by $N=n\cdot M$, where n is the number of H⁺ or OH⁻ ions. This framework is essential for understanding acid-base reactions and titrations in chemistry.

Acid-Base Equivalents Concept 1

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.

Acid-Base Equivalents Concept 2

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.

Acid-Base Equivalents Example 1

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.

Acid-Base Equivalents Concept 3

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.

Acid-Base Equivalents Example 2

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.

Acid-Base Equivalents Concept 4

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.

Acid-Base Equivalents Example 3

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

Calculate mass (grams) needed for the following base equivalent: 0.18 mEq of Mg(OH)₂.

• A. 5.3×10^–3 g
• B. 0.010 g
• C. 10 g
• D. 5.3 g

Answer: (A)

Identify the acid that possesses an equivalent weight of 63 grams.

• A. H₂C₂O₄
• B. HCl
• C. HNO₃
• D. H₂CO₃

Answer: (C)

Determine volume (mL) needed to prepare a 0.73 g of Ca(OH)₂ solution with 1.25 N.

• A. 7.9 mL
• B. 20 mL
• C. 1.6 mL
• D. 16 mL

Answer: (D)

Here’s what students ask on this topic:

What is the definition of equivalents in acids and bases?

Equivalents in acids and bases refer to the measure of the number of $H_{+}$ ions in acids and ${\mathrm{OH}}_{-}$ ions in bases. One equivalent is defined as the amount of acid or base that contributes 1 mole of these ions. For example, 1 mole of hydrobromic acid (HBr) contains 1 mole of $H_{+}$ ions, so it has 1 equivalent. In contrast, 1 mole of calcium hydroxide (Ca(OH)₂) contains 2 moles of ${\mathrm{OH}}_{-}$ ions, so it has 2 equivalents. This concept helps quantify the reactive capacity of acids and bases in chemical reactions.

How do you calculate the number of equivalents for an acid or base?

To calculate the number of equivalents of an acid or base, multiply the number of moles of the acid or base by $n$, where $n$ is the number of $H_{+}$ ions for acids or ${\mathrm{OH}}_{-}$ ions for bases contributed per mole. The formula is: $\mathrm{Equivalents}=n\times \mathrm{moles}$. For example, calcium hydroxide has $n\; =\; 2$ because it provides 2 hydroxide ions per mole. If you have 0.5 moles of calcium hydroxide, the equivalents would be $2\; \times \; 0.5\; =\; 1$ equivalent. This calculation is essential for understanding acid-base reactions and titrations.

What is equivalent weight and how is it calculated for acids and bases?

Equivalent weight is the mass in grams of one equivalent of an acid or base. It is calculated by dividing the molar mass of the acid or base by $n$, the number of $H_{+}$ ions for acids or ${\mathrm{OH}}_{-}$ ions for bases contributed per mole. The formula is: $\mathrm{Equivalent\backslash \; weight}=\; \backslash frac\{\backslash text\{Molar\backslash \; mass\}\}\{n\}$. For example, sulfuric acid (H₂SO₄) has a molar mass of approximately 98 g/mol and provides 2 $H_{+}$ ions, so its equivalent weight is $\backslash frac\{98\}\{2\}\; =\; 49\backslash \; \backslash text\{g/equiv\}$. This value is useful in stoichiometric calculations involving acid-base reactions.

What is normality and how is it related to molarity in acid-base solutions?

Normality (N) is a measure of concentration that represents the number of equivalents of acid or base per liter of solution. It is related to molarity (M), which is moles of solute per liter of solution, by the formula: $\mathrm{Normality}=\; n\; \times \mathrm{Molarity}$, where $n$ is the number of $H_{+}$ ions for acids or ${\mathrm{OH}}_{-}$ ions for bases contributed per mole. For example, a 1 M solution of sulfuric acid (H₂SO₄) has a normality of 2 N because it provides 2 equivalents of $H_{+}$ ions per mole. Normality is especially useful in titration calculations where the reactive capacity of the solution is important.

What are milliequivalents and how do they relate to equivalents?

Milliequivalents (meq) are a smaller unit used to express equivalents, commonly used in clinical and chemical measurements. One equivalent equals 1,000 milliequivalents. This means: $1\backslash \; \backslash text\{equivalent\}\; =\; 1000\backslash \; \backslash text\{milliequivalents\}$. For example, if a solution contains 0.002 equivalents of an acid, it can also be expressed as 2 milliequivalents. Using milliequivalents allows for easier handling of very small quantities of reactive ions in solutions, especially in medical and biochemical contexts.