Relative Formula Mass

Lab Brief #1

🎯 Your mission

Master the concept. Use it tomorrow.

⚡ The twist

Nature loves to surprise the careful watcher.

What you'll learn

The relative formula mass of a compound is the sum of the relative atomic masses of every atom in its formula. Water is 2 × 1 + 16 = 18; calcium carbonate, CaCO₃, is 40 + 12 + 48 = 100. Everything quantitative in chemistry starts here.

Key words

relative atomic mass
The average mass of an element's atoms compared with one-twelfth of a carbon-12 atom.
relative formula mass
The sum of the relative atomic masses of all atoms in a formula.

Make a hypothesis first

Before you start: write down what you think will happen, and why.

Say your guess out loud — then find out if you were right.

Step 1 of 4 · Read

Think about what you just learned: say what "relative atomic mass" and "relative formula mass" mean in your own words before you start.

Step 2 of 4 · Do

Calculate relative formula masses for ten compounds from the periodic table, including ones with brackets such as Ca(OH)₂ and Al₂(SO₄)₃. Check each total against a partner's independently.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 2

The relative formula mass of CaCO₃ (Ca = 40, C = 12, O = 16) is:

How sure are you?😐 maybe

Question 2 of 2

Relative formula mass is found by:

How sure are you?😐 maybe

Mind = blown

🔬 The more you look, the more nature shows you.

Nice work!

You got 0 of 2 right.

Next: The Mole and Avogadro's Constant All lessons

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✨ More to explore

🌍 Where you see this in real life: This shows up in your everyday life more than you'd think.

🏆 Stretch challenge (try it this week): Try one experiment in real life this week — even a tiny one.

👨‍👩‍👧 For the dinner table: “What's the most surprising thing you learned today?”

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Relative Formula Mass

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