AQA · Chemistry · 8462

GCSE Chemistry (AQA): Relative Formula Mass

Topic 2 — Bonding, Structure & Properties of Matter. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Relative Formula Mass | Mr & Percentage Mass Calculations (2027/28 exams)"; each card deep-links to the moment in the video it came from. Lesson 3.01 of Cognito's AQA GCSE Combined Science (Trilogy) and Chemistry course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-c_3.01.

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  1. What is the percentage mass of oxygen in calcium hydroxide, Ca(OH)₂?

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    43.2%, from 32 ÷ 74 × 100.

  2. Why is the relative atomic mass of chlorine 35.5?

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    It is the abundance-weighted average of the chlorine-35 and chlorine-37 isotopes, so it is not a whole number.

  3. What does an element's mass number tell you?

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    The number of protons plus the number of neutrons in the nucleus. It is shown in the top left of the nuclear symbol.

  4. What is the Mr of sulfuric acid, H₂SO₄? (Ar: H = 1, S = 32, O = 16)

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    98, from (2 × 1) + 32 + (4 × 16).

  5. What is the percentage mass of sulfur in sulfuric acid, H₂SO₄?

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    32.7%, from (32 × 1) ÷ 98 × 100.

  6. What is the Mr of calcium hydroxide, Ca(OH)₂? (Ar: Ca = 40, O = 16, H = 1)

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    74, from 40 + (2 × 16) + (2 × 1).

  7. What is the equation for the percentage mass of an element in a compound?

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    (Ar of the element × number of its atoms) ÷ Mr of the compound, all × 100.

  8. How do you calculate the relative formula mass (Mr) of a compound?

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    Add together the relative atomic masses of every atom in the compound's formula.

  9. What is the Mr of magnesium chloride, MgCl₂? (Ar: Mg = 24, Cl = 35.5)

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    95, from 24 + (35.5 × 2).

  10. How many oxygen atoms are in one formula unit of Ca(OH)₂?

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    Two. The subscript 2 outside the brackets multiplies both the oxygen and the hydrogen inside them.

  11. Define relative atomic mass (Ar).

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    The average mass of all the isotopes of an element, weighted by how abundant each isotope is.

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