Mole concept: grams, moles, particles and gas volume

Pick a substance and an amount in moles and see the mass, the number of molecules and atoms, and the volume of a gas at STP, with the mole shown as a pile of 6.022 × 10²³ particles. Free for every student.

Also searched as: mole concept class 11 · mole concept class 11 · mole concept · avogadro number · molar mass · moles to grams

About this experiment

Aim

To understand the mole as a counting unit, and to convert between mass, amount in moles, the number of particles and the volume of a gas at STP.

Theory

Atoms and molecules are far too small to count one by one, so chemists count them in groups. One mole is the amount of a substance that contains 6.022 × 10²³ elementary particles (atoms, molecules or ions). This number is the Avogadro constant, N_A. It is chosen so that the mass of one mole of a substance in grams equals its atomic or molecular mass in u.

The mass of one mole is the molar mass M (in g/mol): 18.015 g/mol for water, 44.01 g/mol for CO₂, 58.44 g/mol for NaCl. The amount in moles is n = m/M, the number of particles is N = n × N_A, and the number of atoms is N times the atoms in one molecule (a water molecule has 3 atoms).

Equal volumes of all gases under the same conditions contain the same number of molecules (Avogadro's law). At STP (0 °C and 1 atm) one mole of any ideal gas occupies 22.4 litres, so V = n × 22.4 L. This works for gases only, not for liquids or solids.

Formulas used

• n = m / M
• N = n × N_A, N_A = 6.022 × 10²³ mol⁻¹
• number of atoms = N × (atoms per molecule)
• V(STP) = n × 22.4 L (gases)
• m = n × M

Procedure: what to do in the virtual lab

1. Choose water and set n = 1 mol. Read the mass (18 g) and the number of molecules.
2. Change the substance to CO₂ and then to NaCl and see how the same one mole has very different masses but the same number of particles.
3. Choose a gas and look at the box: 1 mole fills 22.4 L at STP, whatever the gas is.
4. Set n = 0.5 and 2 mol and check that mass, particles and volume all scale in proportion.

Observations to make

• The pile has the same number of spheres for every substance with the same number of moles: only the mass changes.
• The slope of the graph is the molar mass: glucose has the steepest line.
• Liquids and solids show no gas volume: the 22.4 L rule is for gases only.

What you can change in this experiment

• Substance: Water (H₂O) / Carbon dioxide (CO₂) / Oxygen (O₂) / Hydrogen (H₂)
• Amount of substance (mol)

Viva questions on Mole concept

Questions an examiner may ask in the Class 11 chemistry practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

1. What is a mole? What is Avogadro's number?
2. How many molecules are present in 9 g of water?
3. What is the volume of 2 mol of oxygen at STP?
4. What is the difference between molar mass and molecular mass?

Frequently asked questions

Syllabus: Class XI Chemistry, Chapter 1 — Some Basic Concepts of Chemistry, §1.7 (laws of chemical combination), §1.8 (mole concept and molar masses)

What you do in the lab

Change the controls and watch the chemistry behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: mole concept · Avogadro number · molar mass · molar volume · moles to grams · number of molecules · some basic concepts of chemistry

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