I wanted to know why molarity is temperature-dependent whereas molality is temperature-independent.
$\begingroup$
$\endgroup$
5
-
2$\begingroup$ Density is temperature dependent, but mass is not. So a volume of liquid solvent changes with temperature whereas a mass of solvent does not. $\endgroup$– Ed VCommented Jul 10, 2020 at 12:26
-
$\begingroup$ So then what is the temperature at which densities are calculated? $\endgroup$– Shreyansh KuntalCommented Jul 10, 2020 at 12:28
-
$\begingroup$ Any temperature. Generally, densities are measured at chosen temperatures and then the dependence is approximated by an empirical function ( as it is smooth enough ) and eventually tabelated. $\endgroup$– PoutnikCommented Jul 10, 2020 at 12:33
-
$\begingroup$ The glassware, e.g., volumetric flash, pipette, etc., are calibrated by the manufacturer and the temperature is typically either on the glassware or in accompanying documentation. Commonly the temperature is 20 degrees C. $\endgroup$– Ed VCommented Jul 10, 2020 at 12:35
-
$\begingroup$ @Shreyansh Kuntal Your last question should rather be: "So then what is the temperature at which volumes are measured? ", as this is the one Ed V has answered. $\endgroup$– PoutnikCommented Jul 10, 2020 at 12:47
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
This happens because molarity is number of moles per volume of solution and molality is number of moles per mass of the solvent. Now, volume is temperature dependent property since most substances expand with increase in temp and vice versa. Hence the ratio of moles to volume change with temperature. But mass is a temperature independent property hence it does not change with temperature unless the substance or the solvent is getting evaporated constantly due to the conditions in which it is kept. Hence molarity is temperature dependent whereas molality is not.
