Why is 1 molar aqueous solution more concentrated than a 1 molal solution?

A 1 molar (1 M) aqueous solution is more concentrated than a 1 molal (1 mol/kg) solution because they measure concentration in different ways.

Molarity (M) is a measure of concentration that expresses the number of moles of solute per liter of solution. So, a 1 M solution contains 1 mole of solute dissolved in 1 liter of solvent (water in this case).

Molality (mol/kg) is a measure of concentration that expresses the number of moles of solute per kilogram of solvent. So, a 1 molal solution contains 1 mole of solute dissolved in 1 kilogram of solvent.

The difference lies in what's being held constant. In molarity, volume remains constant (usually 1 liter), while in molality, mass (of the solvent) remains constant.

Consider a scenario where you dissolve 1 mole of solute in 1 liter of water (for molarity) and in 1 kilogram of water (for molality):

For the 1 M solution, you have 1 mole of solute in 1 liter of water.
For the 1 molal solution, you still have 1 mole of solute, but now it's dissolved in 1 kilogram of water.
Since 1 liter of water has less mass than 1 kilogram of water, the 1 M solution has a higher concentration of solute per unit volume compared to the 1 molal solution. Therefore, the 1 M solution is more concentrated.