Molarity vs. Molality: What's the Difference?
In chemistry, there are multiple ways to define "concentration". The two most easily confused are Molarity (M) and Molality (m).
Molarity (M)
Definition: Moles of solute per Liter of Solution.
M = mol solute / L solution
- Symbol: M (pronounced "Molar")
- Use Case: Most general lab solutions (buffers, media).
- Temperature Dependent? YES. Volume changes with temperature (expansion/contraction), so Molarity changes slightly with T.
Molality (m)
Definition: Moles of solute per Kilogram of Solvent.
m = mol solute / kg solvent
- Symbol: m (pronounced "Molal")
- Use Case: Physical chemistry properties (boiling point elevation, freezing point depression).
- Temperature Dependent? NO. Mass does not change with temperature.
Which one should I use?
For 99% of biology and wet lab chemistry applications, you want Molarity.
- "Make a 1M NaCl solution" means: Dissolve 58.44g NaCl in slightly less than 1L water, stir, then top up to exactly 1L.
Calculate Molarity Instantly
Stop doing mental math. Use our calculator to find the exact mass needed.
Example Calculation
Task: Prepare 500 mL of 0.5 M NaOH (MW = 40 g/mol).
- Formula:
Mass = Conc (M) * Vol (L) * MW - Plug values:
0.5 * 0.5 * 40 - Result:
10 grams
Dissolve 10g NaOH in ~400mL water, let it cool (it's exothermic!), then top up to 500mL.