Question

Why is $1M$ more concentrated than $1m$ ?

Answer 1

We can define molarity as the number of moles of solute present in one litre of solution, whereas molality is defined as the number of moles of solute present in one kilogram (kg) of solvent. Both molarity and molality are the units of concentration.

Complete answer:
We have seen that molarity and molality both are units of concentration of the solution.
Now, coming to our question;
$1M$ (1 molar) solution will contain one mole of solute in one litre of solution. Here both solute and solvent make up one litre $\left( {1000g} \right)$
$1m$ (1 molal) solution will contain one mole of solute in one kilograms $\left( {1000g} \right)$ of solvent. Here only solvent make up the $1000g$
Thus, it can be seen that a $1M$ solution is more concentrated than a $1m$ solution.

Additional information: The important point to note is that both molarity and molality are the units of concentration of solution, usually it doesn’t matter which unit we use but molality is preferred over molarity when the solution will undergo temperature changes. This is because temperature changes may alter the volume thus changing the concentration if molarity is used. Therefore, it is advisable to use molality in case of temperature changes.

Note:
We have seen that one molar is more concentrated than one molal. Also, the advantage of molality over molarity is that it is unaffected by changes in temperature and pressure since it is calculated based on mass and not on volume. We can say that molality is a more accurate measure of concentration than molarity.