Q.$20\ \text{g}$ of common salt ($\text{NaCl}$) is dissolved in $180\ \text{g}$ of water to give $200\ \text{g}$ of solution. Calculate the mass percentage of $\text{NaCl}$ in the solution.

Only molarity is defined using the volume of the solution, and volume is the one quantity in these definitions that genuinely changes with temperature.

Molarity, $M=\dfrac{n_{solute}}{V_{solution}(\text{L})}$: as temperature rises, the solution expands (volume $V$ increases), so for the same fixed amount of solute the molarity numerically decreases — molarity is temperature-dependent.

Molality, $m=\dfrac{n_{solute}}{\text{mass of solvent (kg)}}$: mass does not change with temperature, so molality of a given solution stays fixed regardless of temperature.

…

Molarity of pure water = (mass of 1 L of water) / (molar mass of water), using its density of ~1 g/mL.

Calculation: Take 1 litre (1000 mL) of pure water. Since the density of water is ~1 g/mL, this has a mass of 1000 g.

Moles of water $=\dfrac{1000\text{ g}}{18\text{ g/mol}}=55.55\text{ mol}$

Since this is the number of moles present in 1 litre, the molarity is: …

Concentration terms defined using masses (or moles) rather than the solution's volume are temperature-independent, because mass does not change when a liquid expands or contracts on heating/cooling.

Concentration terms like molarity and normality are defined per unit volume of solution. Since the volume of a liquid changes with temperature (thermal expansion/contraction), these values change slightly with temperature even though the actual amount of solute present is unchanged.

By contrast:

• Molality ($m$) = moles of solute per kilogram of solvent (a mass-based ratio).
• Mole fraction ($x$) = moles of one component divided by total moles of all components (a ratio of amounts of substance). …

Molarity is volume-dependent (moles per litre of solution), so it falls as the solution expands on heating; mass-based measures (molality, mole fraction, mass %) are temperature-independent.

On heating an aqueous solution, the volume of the solution increases (thermal expansion) while the number of moles of solute and the total mass remain unchanged.

• Molarity $= \dfrac{\text{moles of solute}}{\text{volume of solution (L)}}$ — since volume increases, molarity decreases. …

The WHO guideline (safe) limit for copper in drinking water is 2 ppm (2 mg/L).

Background

Concentration in parts per million (ppm) expresses the mass of solute per $10^6$ parts of solution — e.g. 1 ppm = 1 mg of solute per litre of water (since 1 L of dilute aqueous solution ≈ 1000 g). This unit is commonly used for trace pollutant/metal-ion levels in drinking water because the concentrations involved are extremely small.

The specific limit …