Explicit science instruction
Molarity is defined as M=n/V, where n is moles of solute and V is total solution volume in litres.
Unit discipline matters: millilitres must be converted to litres before substitution. The denominator is solution volume, not solvent volume.
A concentration value is intensive: doubling both solute amount and final solution volume leaves molarity unchanged.
Vocabulary: molarity; mole; litre; concentration; intensive property.
Worked example with reasoning
A solution contains 0.50 mol solute in 2.00 L solution. M=0.50/2.00=0.250 mol/L.
Guided question
How many moles are present in 125 mL of a 0.40 M solution? Show the mL→L conversion.
Independent work
- Solve four molarity problems for M, n, or V.
- Perform a unit audit on each.
- Explain why 1 mol in 1 L and 2 mol in 2 L have the same molarity.
Misconception check
Using millilitres directly in M=n/V gives a result off by a factor of 1000.
Formative understanding check
What volume must appear in the molarity denominator?
Reveal response
The final total solution volume expressed in litres.
Mentor evidence conference
Ask Archaemenes to check unit conversions, model choice, and whether the calculation matches the particle story.
Teacher guidance
Foundation: Use a formula triangle and unit-conversion scaffold.
Core: Solve mixed molarity problems with units.
Extended: Analyze uncertainty when solution volume is reported with limited precision.