GCSE Chemistry - How to Find the Volume of a Gas #28

Summary

This video explains how to calculate the volume of gases using a simple equation that multiplies the number of moles by 24, which works for all gases at room temperature and pressure. It covers examples like chlorine, oxygen, and water vapor, and discusses rearranging the equation to find moles from volume. The video also addresses finding gas volumes from chemical reactions using molar ratios, and highlights shortcuts when only converting between gas volumes. The method applies under standard conditions, emphasizing its use in exams.

Takeaways

• 📏 The equation for calculating the volume of a gas (in dm³) is simple: multiply the number of moles by 24.
• 🧪 The type of gas (e.g., chlorine, water vapor, oxygen) doesn't matter in this equation.
• 🔢 Example: 3.5 moles of chlorine gas takes up 84 dm³ of volume (3.5 x 24).
• 🔄 The equation can be rearranged to calculate moles if the volume is known (volume ÷ 24 = moles).
• 💧 Example: 60 dm³ of oxygen equals 2.5 moles of oxygen (60 ÷ 24 = 2.5).
• 💡 For mass calculations, use the formula mass ÷ Mr (molar mass) to find moles, then use the gas equation to find volume.
• 🚰 Example: 27 grams of water vapor with an Mr of 18 equals 1.5 moles, which occupies 36 dm³ of volume (1.5 x 24).
• 🧫 When reacting gases, the volume of the product depends on the limiting reactant, in this case, nitrogen.
• 🔗 Use molar ratios from the balanced equation to calculate product volumes (e.g., 18 dm³ nitrogen forms 36 dm³ ammonia).
• ⚠️ The 24 constant only works for gases at room temperature and pressure; changes in temperature or pressure would affect the gas volume.

Q & A

What is the basic equation used to calculate the volume of a gas?

The volume of a gas can be calculated by multiplying the number of moles of the gas by 24, as long as the gas is at room temperature and pressure. The result is the volume in decimeters cubed.

Does the type of gas affect the calculation of volume using this equation?

No, the type of gas does not affect the calculation. Whether it’s chlorine, water vapor, or oxygen, the same equation can be applied.

How would you calculate the volume of chlorine gas if you have 3.5 moles of it?

To find the volume of 3.5 moles of chlorine gas, you multiply the number of moles (3.5) by 24. The result is 84 decimeters cubed.

How can the equation be rearranged to find the number of moles from the volume of gas?

To find the number of moles from the volume, the equation can be rearranged to 'volume ÷ 24 = moles.' For example, if you have 60 dm³ of oxygen, you divide 60 by 24 to get 2.5 moles of oxygen.

How do you find the volume of a gas if you're given its mass instead of moles?

To find the volume from mass, first calculate the number of moles using the equation 'moles = mass ÷ relative formula mass (Mr).' Once you have the moles, multiply them by 24 to find the volume in decimeters cubed.

What is the process for finding the volume of water vapor if you are given 27 grams of it?

First, calculate the moles of water vapor by dividing the mass (27 grams) by the relative formula mass of water (18). This gives 1.5 moles. Then multiply 1.5 moles by 24 to get a volume of 36 dm³.

How do you calculate the volume of ammonia produced from 18 dm³ of nitrogen in a reaction with excess hydrogen?

First, calculate the moles of nitrogen by dividing 18 dm³ by 24, which gives 0.75 moles. Using the molar ratio of nitrogen to ammonia (1:2), you find that 0.75 moles of nitrogen will produce 1.5 moles of ammonia. Multiply 1.5 by 24 to get 36 dm³ of ammonia.

Can you simplify the process of calculating gas volumes when only gas volumes are involved?

Yes, if you're only converting between gas volumes, you can use the molar ratio directly without finding moles. For instance, if the ratio of nitrogen to ammonia is 1:2, you can multiply the volume of nitrogen by 2 to get the volume of ammonia.

How much hydrogen would react with 4 dm³ of nitrogen based on the molar ratio?

Since the molar ratio of nitrogen to hydrogen is 1:3, you multiply the nitrogen volume (4 dm³) by 3 to get 12 dm³ of hydrogen.

Does this equation with the number 24 work at any temperature and pressure?

No, this equation with the number 24 only works for gases at room temperature and pressure. Changing the temperature or pressure would alter the gas volume.