Acid-Base Equivalents - Online Tutor, Practice Problems & Exam Prep

Equivalents measure the amount of H⁺ or OH^- ions in acids and bases. The equivalent of an acid is the amount that contributes 1 mole of H⁺ ions, while for a base, it’s 1 mole of OH^- ions. Equivalent weight is calculated as the molar mass divided by n, where n is the number of H⁺ or OH^- ions. Normality, representing equivalents per liter, is calculated using the formula: N=n×M, linking it to molarity.

Acid-Base Equivalents Concept 1

We can say that equivalents are used to measure the number of H⁺OH^- ions in acids and bases respectively. We're going to say when it comes to the equivalent of an acid, it's the amount of acid that contributes 1 mole of acidic H⁺ ions. And equivalence of a base is the amount of base that contributes 1 mole of OH^- ions. For example, we have 1 mole of hydrobromic acid. It possesses in it 1 H⁺ ion. That 1 H⁺ ion is equal to its one equivalent. Here we have 1 mole of calcium hydroxide. It has 2 hydroxides in it, so that's 2 equivalents of calcium hydroxide.

Now we'll go into further detail where what if the moles of the acid and base are different, that can help vary the number of equivalents. So click on the next video. Let's continue talking about, equivalence when it comes to acids and bases.

Acid-Base Equivalents Concept 2

Now to calculate the number of equivalents of acid or base, we simply multiply n by the number of moles of acid or base, n=moles of H+ or OH−. Now with this whole concept, we also have milliequivalents. It's the common unit used to express equivalence. Remember that one equivalent is equal to 1,000 milliequivalents. So if we're talking about acid equivalents, remember, an equivalent is equal to n, which is just the number of H+ ions, times the moles of the acid. And for base, its equivalence is equal to n, which is the number of OH− ions, times the moles of the base. Keep this in mind when looking at the equivalents for either acids or bases.

Acid-Base Equivalents Example 1

Calculate the number of equivalents in each of the following. Oh, so for the first one we have here is 1 mole of phosphoric acid. So this is an acid, and remember it is \( n \), which is the number of \( H^+ \) ions for the acid, times the moles of the acid. Here phosphoric acid has in it, 3 \( H^+ \) ions. So \( n \) equals 3 times, we're gonna say here 1 mole, so that equals 3 equivalents for phosphoric acid.

Now let's look at the base. Here we have \( \text{RbOH} \), which is a base. So the equivalence is equal to \( n \), which is the moles of number of \( OH^− \) times its moles. We know how many \( OH^− \) we have in this compound, we just have just 1. So \( n \) is 1. What we have to do now is figure out the moles of this particular base. So we're gonna have to convert grams to moles. Taking into account, we have 1 Rb, 1 oxygen, 1 hydrogen. So that's 1 mole. Looking up their masses on the periodic table and adding them together gives me a molar mass of 102.476 grams. Grams cancel out and now we're gonna have 0.026 moles of this base. So that number goes here. So one times that number means I have 0.026 equivalents of my base. So this is how you'd identify and calculate the equivalence of either an acid or a base.

Acid-Base Equivalents Concept 3

So the equivalent weight represents the mass in grams of 1 acid or base equivalent. Here we're going to say equivalent weight equals the molar mass of the acid or base divided by n . For an acid, n equals the number of H+ ions. For base, it equals the number of OH- ions.

Acid-Base Equivalents Example 2

Here we need to calculate the equivalent weight of sulfuric acid. So remember, equivalent weight equals the molar mass of the acid in this case, divided by n, which is the total number of H+ ions. So, sulfuric acid has 2 H+ ions, so n will be 2. And then, the molar mass is just calculating the combined mass of the 2 hydrogens, the 1 sulfur, and the 4 oxygens taken from the periodic table. If you do that correctly, you'd have 98.086 grams per mole as the molar mass for sulfuric acid. So that number gets divided by 2, resulting in 49.043 grams per mole as the equivalent weight of sulfuric acid.

Acid-Base Equivalents Concept 4

The concentration of acid or base in aqueous solutions is represented by normality. Now normality itself represents the number of equivalents per liter of solution. And recall that molarity itself equals moles over liters, moles of solute over liters of solution. Here, we can use this information to apply it to normality. Now normality itself, we said, can equal equivalence per liters of solution, or we can say that normality equals n. Remember, n for acids is the number of H+ ions, n for bases is the number of OH- ions, times your molarity, which is capital M. So remember, these are the two formulas we can use to establish relationships with normality of solutions.

Acid-Base Equivalents Example 3

Calculate the normality of each of the following solutions.

For a, we have \(4.6 \times 10^{-2}\) molar of sodium hydroxide. Recall that normality is simply \(n \times \)molarity, where \(n\) is the number of \(OH^-\) ions for the base. Since there's only one \(OH^-\) in sodium hydroxide, it will be \(1 \times 4.6 \times 10^{-2}\). Therefore, the normality will be \(4.6 \times 10^{-2}\).

For b, a bit more calculation is required. Here, we have 0.35 grams of phosphoric acid in 1 liter. We use the equation: N = e q u i v a l e n c e l i t e r s of solution. With 1 liter on the bottom, we need to determine the equivalents of this base. Recall that for a base, the equivalents are \(n \times\) moles. \(H_3PO_4\) has 3 \(H^+\) ions, hence \(n = 3\). We now need the moles of phosphoric acid, where 0.35 grams are provided. Given a molar mass of 97.994 for phosphoric acid (3 hydrogens, 1 phosphorus, and 4 oxygens), we calculate the moles which is approximately \(0.0035716\) moles. This gives \(0.0107148\) equivalents when multiplied by 3. Dividing by 1 liter results in a normality of approximately \(0.011\), considering significant figures based on the mass given (0.35 has 2 significant figures).

Thus, the answers for parts a and b are \(4.6 \times 10^{-2}\) and \(0.011\) normality respectively.

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

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

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

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

Here’s what students ask on this topic:

What is the definition of an equivalent in acid-base chemistry?

An equivalent in acid-base chemistry measures the amount of H⁺ or OH^- ions in acids and bases, respectively. For an acid, one equivalent is the amount that contributes 1 mole of H⁺ ions. For a base, one equivalent is the amount that contributes 1 mole of OH^- ions. This concept helps in quantifying 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 for an acid or base, you use the formula: $n\times \mathrm{moles}$, where $n$ is the number of H⁺ ions for acids or OH^- ions for bases. For example, if you have 1 mole of H₂SO₄ (which has 2 H⁺ ions), the number of equivalents is 2.

What is equivalent weight and how is it calculated?

Equivalent weight represents the mass in grams of one equivalent of an acid or base. It is calculated using the formula: $\frac{\mathrm{molar} \mathrm{mass}}{n}$, where $n$ is the number of H⁺ ions for acids or OH^- ions for bases. For example, the equivalent weight of H₂SO₄ (molar mass = 98 g/mol) is 49 g/equiv because it has 2 H⁺ ions.

What is normality and how is it related to molarity?

Normality (N) is a measure of concentration that represents the number of equivalents per liter of solution. It is related to molarity (M) through the formula: $N=n\times M$, where $n$ is the number of H⁺ ions for acids or OH^- ions for bases. For example, a 1 M solution of H₂SO₄ (which has 2 H⁺ ions) has a normality of 2 N.

What are milliequivalents and how are they used in acid-base chemistry?

Milliequivalents (mEq) are a common unit used to express equivalents in smaller quantities. One equivalent is equal to 1,000 milliequivalents. They are used in acid-base chemistry to measure the reactive capacity of acids and bases in smaller, more precise amounts. For example, if you have 0.5 equivalents of an acid, it is equal to 500 milliequivalents.