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Moles, Mass & RFM

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2 guided lessons · 18 checks · 1. Principles of Chemistry → Chemical Formulae, Equations, Calculations

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The Mole

Bundles make counting practical. Paper is sold in reams and eggs in dozens, and chemists bundle particles in the same spirit: one mole is a fixed count of particles, 6.02 × 10^23 of them. Because every mole holds the same count, one mole of different substances generally has a different mass — a value called the molar mass.

What you know already

You already met relative atomic mass (Ar) in Atomic Structure, where it compares the mass of an atom with the carbon-12 scale. You can also write formulae such as H2O and NaCl and add atomic masses to find a compound's relative formula mass. This note adds the mole, the bridge between the masses you can weigh and the numbers of particles that react.

Key points of understanding

Chemical amounts are counted in moles, and the mole (mol) is the accepted unit of amount of substance.
Equal amounts in moles contain equal numbers of particles, whatever the substance.
The number of particles in one mole is the Avogadro constant, 6.02 × 10^23 per mole.
The mass of one mole is the molar mass: for an element it matches the relative atomic mass in grams.
For a compound, the molar mass matches the relative formula mass in grams, found by adding the atomic masses together.

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1.The Mole

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Key points

Chemical amounts are counted in moles, and the mole (mol) is the accepted unit of amount of substance.
Equal amounts in moles contain equal numbers of particles, whatever the substance.
The number of particles in one mole is the Avogadro constant, 6.02 × 10^23 per mole.
The mass of one mole is the molar mass: for an element it matches the relative atomic mass in grams.
For a compound, the molar mass matches the relative formula mass in grams, found by adding the atomic masses together.

First pass

A mole is simply a fixed number of particles: 6.02 × 10^23 of them, the same count for every substance. The mass of that count is the molar mass, and for an element it equals the relative atomic mass written in grams.

Analogy — Think of a dozen. A dozen eggs and a dozen oranges hold the same count but weigh differently; a mole of carbon and a mole of helium also hold the same count but weigh 12 g and 4 g.

Exam pointer — Questions may ask how many particles sit in a mole of atoms, molecules, ions or electrons — the count is identical, so always name the particle type in the answer.

Going deeper

The mole (mol) is the unit of amount of substance: one mole of any species contains 6.02 × 10^23 of the particles stated, whether atoms, molecules, ions or formula units. Molar mass links that count to grams, matching Ar for elements and Mr for compounds.

Analogy — A bank counting notes in bundles still cares which currency it holds; a chemist counting particles in moles still cares whether the bundle contains sodium atoms or hydrogen molecules.

Exam pointer — Expect a comparison item such as: one mole of carbon holds as many atoms as there are molecules in one mole of carbon dioxide — state the particle type, then apply the Avogadro constant.

Checks

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1.Which quantity does one mole measure?
2.How many atoms are present in one mole of helium?
3.One mole of carbon and one mole of helium are compared. Which statement is true?
4.What is the mass of one mole of lithium? (Ar: Li = 7)
5.Which value is the molar mass of water, H2O? (Ar: H = 1, O = 16)
6.State the value of the Avogadro constant.
Written response · recall
Your answer
7.State what the molar mass of an element matches, in grams.
Written response · recall
Your answer
8.Explain why one mole of carbon and one mole of helium contain the same number of particles.
Written response · explain why
Your answer
9.A student needs the molar mass of carbon dioxide, CO2 (Ar: C = 12, O = 16). State how it is found from the atomic masses.
Written response · application
Your answer

2.Moles-Mass Calculations

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Balances cannot weigh out one mole directly, yet reactions happen particle by particle in fixed molar ratios. The way through is arithmetic: one relationship links moles, mass in grams and molar mass, and it works in both directions. Master the rearrangement once and every moles-mass question becomes two lines of working.

The Moles & Mass Formula Triangle — Formula triangle for moles, mass and molar mass

Key points

Reactions happen in molar ratios, but laboratory balances read grams, so converting between the two units is unavoidable.
One relationship links the quantities: moles = mass ÷ molar mass.
Rearranged the same way, mass = moles × molar mass.
Molar mass carries the unit g mol^-1 and is numerically equal to Ar for an element or Mr for a compound.
Standard form keeps very small or large answers readable, such as 7.71 × 10^-3 mol for 2.64 g of sucrose.

First pass

To find moles, divide the mass by the molar mass; to find mass, multiply the moles by the molar mass. Watch the units as a direction check: g mol^-1 cancels grams to leave moles.

Analogy — Coins come in rolls of a fixed mass. Weighing a bag of rolls and dividing by the mass of one roll tells you how many rolls you have — the same move as grams divided by g mol^-1.

Exam pointer — Write the relationship before substituting numbers and show every step; method marks survive an arithmetic slip, while a bare answer does not.

Going deeper

Amount in moles = mass in grams ÷ molar mass in g mol^-1, where molar mass equals Ar for an element and Mr for a compound; rearranging gives mass = moles × molar mass. Quote very small or very large answers in standard form to three significant figures.

Analogy — A currency converter uses one exchange rate in two directions, pounds to euros or the reverse; the moles-mass relationship is that single rate, and molar mass is the published figure on the board.

Exam pointer — Stems often supply Mr directly, as with 2.64 g of sucrose at Mr 342.3; divide, then present the answer in standard form with the unit mol still visible.