OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
S.I. units: (mol)
The symbol (mol) is the S.I. unit for amount of substance.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 16
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The symbol (mol) is the S.I. unit for amount of substance. One mole contains the Avogadro constant, approximately 6.02 × 10²³, of specified particles such as atoms, molecules or ions. Amount of substance is not mass: the mass of one mole depends on the substance's molar mass. For example, 1 mol of carbon-12 has a mass of 12 g, while 1 mol of oxygen molecules (O₂) has a mass of about 32 g. To find the number of moles, divide mass in grams by molar mass in g mol⁻¹, or divide number of particles by the Avogadro constant. In equations, the mole allows you to count particles by weighing.
What earns the marks
- The mole (mol) is the S.I. base unit of amount of substance.
- One mole contains the Avogadro constant of particles, approximately 6.02 × 10²³ mol⁻¹.
- Amount of substance is not the same as mass.
- The molar mass of a substance is the mass per mole, usually in g mol⁻¹.
- Number of moles equals mass divided by molar mass.
- Number of moles equals number of particles divided by the Avogadro constant.
Common mistakes and how to put them right
Thinking that one mole of any substance has the same mass.
Put it right: The mass depends on the molar mass, so 1 mol of O₂ has a greater mass than 1 mol of C.
Using the mole as a unit of mass.
Put it right: It is the unit of amount of substance, and mass is measured in kilograms or grams.
Forgetting to convert mass from grams to kilograms when using S.I. units in equations.
Put it right: Divide grams by 1000 to get kilograms.
Confusing the Avogadro constant with the number of moles.
Put it right: The constant is the number of particles per mole, not the amount itself.
Exam tips
- In multiple-choice questions, check whether the answer requires moles, mass or number of particles; use the correct relationship.
- When using n = m / M, ensure mass is in grams if molar mass is in g mol⁻¹, or convert both to S.I. units consistently.
- Remember that the mole is a base unit, so it cannot be expressed as a combination of other base units.
Try the lesson
The start of the S.I. units: (mol) lesson, made from checked teaching notes for this specification point.
Listen
The symbol (mol) is the S.I. unit for amount of substance. One mole contains the Avogadro constant, approximately 6.02 × 10²³, of specified particles such as atoms, molecules or ions. Amount of substance is not mass: the mass of one mole depends on the substance's molar mass. For example, 1 mol of carbon-12 has a mass of 12 g, while 1 mol of oxygen molecules (O₂) has a mass of about 32 g. To find the number of moles, divide mass in grams by molar mass in g mol⁻¹, or divide number of particles by the Avogadro constant. In equations, the mole allows you to count particles by weighing. A strong answer earns marks for these points. The mole (mol) is the S.I. base unit of amount of substance. One mole contains the Avogadro constant of particles, approximately 6.02 × 10²³ mol⁻¹. Amount of substance is not the same as mass. The molar mass of a substance is the mass per mole, usually in g mol⁻¹. Exam tip: In multiple-choice questions, check whether the answer requires moles, mass or number of particles; use the correct relationship.
See it
Key words
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Amount of substance
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Avogadro constant
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Molar mass
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Molar calculations
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See every definition
- Amount of substance
- The quantity for which the mole (mol) is the S.I. unit, and which is not the same as mass.
- Avogadro constant
- The number of specified particles contained in one mole, approximately 6.02 × 10²³.
- Molar mass
- The mass per mole of a substance, usually in g mol⁻¹, on which the mass of one mole depends.
- Molar calculations
- Finding the number of moles by dividing mass in grams by molar mass in g mol⁻¹, or by dividing number of particles by the Avogadro constant.
Label the diagram
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