Lesson Plan

• Define relative molecular mass ($M_r$) for neutral molecules.
• Distinguish relative molecular mass from relative formula mass for ionic compounds.
• Calculate $M_r$ for given molecular or empirical formulas using atomic masses.
• Convert between mass and moles using the relationship $moles = \frac{mass}{M_r}$.
• Apply $M_r$ in basic stoichiometric calculations and answer related practice questions.

• Projector and screen
• Whiteboard and markers
• Printed worksheet with practice questions
• Periodic‑table handouts showing relative atomic masses
• Scientific calculators
• Molecular model kits (optional)
• Answer key for teacher

Begin with the question “How could we compare the mass of a water molecule to a carbon‑dioxide molecule without a balance?” Students recall that they already know atomic symbols and relative atomic masses. Explain that today’s success criteria are to correctly compute $M_r$ for both molecular and ionic compounds and to use it to convert mass to moles.

1. Do‑now (5 min): Quick mental quiz on relative atomic masses of common elements.
2. Mini‑lecture (10 min): Introduce definitions of $M_r$, show formulae, work through water and CO₂ examples on the board.
3. Guided practice (10 min): Students calculate $M_r$ for NaCl and MgO while the teacher models the steps.
4. Independent practice (10 min): Worksheet – calculate $M_r$ for NH₃, CaCO₃ and solve the mass‑to‑moles question for H₂O.
5. Check for understanding (5 min): Exit ticket – give a formula (e.g., C₆H₁₂O₆); students write the correct $M_r$.

Summarise that $M_r$ provides a quick way to move between mass and moles, and that the same symbol is used for both molecular and formula masses but with different contexts. Collect exit tickets to gauge understanding, and assign homework: complete additional $M_r$ calculations for a set of given compounds.