What is Henry's Law?

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Grade Level:

Class 12

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Definition

What is it?

Henry's Law tells us that the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas above the liquid. Simply put, the more pressure a gas has above a liquid, the more of it will get 'pushed' into the liquid and dissolve.

Simple Example

Quick Example

Imagine you're opening a bottle of fizzy drink like a soda or a Thums Up. Before you open it, there's a lot of carbon dioxide gas dissolved in the drink because it's bottled under high pressure. The moment you open the cap, you hear a 'hiss' and see bubbles forming. This is because the pressure above the liquid drops, and according to Henry's Law, less gas can stay dissolved, so it escapes.

Worked Example

Step-by-Step

Let's say the solubility of a gas (Gas X) in water is 0.05 mol/L when its partial pressure above the water is 2 atm. What will be its solubility if the partial pressure is increased to 4 atm?

Step 1: Understand Henry's Law: S = kH * P, where S is solubility, P is partial pressure, and kH is Henry's Law constant.
---Step 2: Use the given initial conditions to find Henry's Law constant (kH). We have S1 = 0.05 mol/L and P1 = 2 atm.
0.05 mol/L = kH * 2 atm
kH = 0.05 mol/L / 2 atm = 0.025 mol/(L·atm)
---Step 3: Now use the calculated kH and the new partial pressure (P2 = 4 atm) to find the new solubility (S2).
S2 = kH * P2
S2 = 0.025 mol/(L·atm) * 4 atm
---Step 4: Calculate the final solubility.
S2 = 0.10 mol/L

Answer: The solubility of Gas X will be 0.10 mol/L when the partial pressure is 4 atm.

Why It Matters

Henry's Law is crucial for understanding how gases behave in liquids, which is vital in fields like medicine for blood gas analysis, in engineering for designing scuba gear, and in environmental science for studying oxygen levels in water bodies. Knowing this helps doctors, engineers, and scientists solve real-world problems.

Common Mistakes

MISTAKE: Thinking Henry's Law applies to all types of solutions, even solids dissolving in liquids. | CORRECTION: Henry's Law specifically deals with the solubility of GASES in liquids, not solids.

MISTAKE: Confusing partial pressure with total pressure. | CORRECTION: Henry's Law uses the PARTIAL pressure of the specific gas that is dissolving, not the total pressure of all gases present.

MISTAKE: Believing that increasing temperature always increases gas solubility. | CORRECTION: For most gases, increasing temperature actually DECREASES their solubility in liquids, which is the opposite effect of increasing pressure.

Practice Questions

Try It Yourself

QUESTION: If the partial pressure of oxygen gas above water is doubled, what happens to its solubility in the water (assuming temperature is constant)? | ANSWER: Its solubility will also double.

QUESTION: A gas has a Henry's Law constant of 0.015 mol/(L·atm) at 25 degrees C. If the partial pressure of this gas is 3 atm, what is its solubility in mol/L? | ANSWER: Solubility = 0.015 * 3 = 0.045 mol/L

QUESTION: The solubility of nitrogen gas in water at 25 degrees C and 1 atm pressure is 6.8 x 10^-4 mol/L. Calculate the Henry's Law constant for nitrogen. Then, find the solubility of nitrogen if its partial pressure is 0.8 atm. | ANSWER: First, kH = S/P = (6.8 x 10^-4 mol/L) / 1 atm = 6.8 x 10^-4 mol/(L·atm). Then, S = kH * P = (6.8 x 10^-4 mol/(L·atm)) * 0.8 atm = 5.44 x 10^-4 mol/L.

MCQ

Quick Quiz

Which of the following statements about Henry's Law is correct?

It states that increasing temperature increases gas solubility.

It applies only to solids dissolving in liquids.

It relates the solubility of a gas to its partial pressure.

It states that pressure has no effect on gas solubility.

The Correct Answer Is:

Henry's Law directly states that the solubility of a gas in a liquid is proportional to its partial pressure above the liquid. Options A, B, and D describe incorrect relationships or applications.

Real World Connection

In the Real World

Scuba divers use Henry's Law every time they dive deep into the ocean. As they go deeper, the pressure increases, and more nitrogen gas from their breathing air dissolves into their blood. If they ascend too quickly, the pressure drops rapidly, causing the dissolved nitrogen to form bubbles in their blood, leading to a painful and dangerous condition called 'the bends'. Doctors and dive instructors use this law to teach safe diving practices.

Key Vocabulary

Key Terms

SOLUBILITY: The maximum amount of a substance that can dissolve in a given amount of solvent at a specific temperature and pressure. | PARTIAL PRESSURE: The pressure that a single gas in a mixture of gases would exert if it alone occupied the same volume. | HENRY'S LAW CONSTANT (kH): A proportionality constant unique to each gas-solvent pair at a specific temperature. | DISSOLVE: To become incorporated into a liquid to form a solution.

What's Next

What to Learn Next

Great job understanding Henry's Law! Next, you should explore Raoult's Law, which explains how adding a solute affects the vapor pressure of a solvent. This will help you understand more complex solution properties and how they are used in chemistry and industry.