Question

A $20\%$ of NaOH in water, $1$molal is more concentrated or$1$molar?

Answer 1

Molarity is defined as moles of the solute per liter of the solution. In this case solute is NaOH and solvent is water. On the other hand, molality is defined as no of moles of solute per kilogram of the solvent. $1$ Molar (M) NaOH will contain $40$g of NaOH in $1000$ml of water and $1$molal (m) NaOH will contain $40$g of NaOH, which is dissolved in $1000$g of water.
$40$ $1000$

Complete answer:
For preparing $1$ molar of NaOH solution, $40$g of NaOH is weighed (equal to $1$ mole) and then water is added to it to make the solution equal to $1000$ml or $1$liter.
But for $1$ molal solution, $40$g of NaOH is weighed and after this $1000$g or $1$kg of solvent i.e. water is weighed separately and then it is mixed with $1$ mole of NaOH. So, basically there is more concentration of water in $1$molal solution as compared to $1$molar solution because in $1$molal solution water is weighed equal to $1$kg separately and then added to the solute while in $1$ molar, solvent is directly added to $1$ mole of solute to make the whole solution equal to $1000$ml.
Since concentration of water is more in $1$ molal so it will be less concentrated in terms of NaOH.
Therefore, $1$ molar solution is more concentrated than $1$ molal solution.

Additional Information: Molar solution will be more concentrated than molal solution when water is used as a solvent. But if solvent is denser than water, then molal (m) solution will be stronger than molar solution, ex- Ethyl acetate.
Molarity of the solution decreases with increase in temperature because volume of the solution increases but the no. of moles of solute remains the same. Molarity is therefore inversely proportional to the temperature because molarity is dependent on volume and volume increases with temperature. If temperature is increased, volume also gets increased which decreases the amount of moles per liter.
Molality does not depend on temperature because neither the no. of moles nor the mass of solvent changes with the change in temperature.

Note:
In molarity the solution is made $1000$ml including both the solute and the solvent whereas in molality the solvent is measured separately as $1000$g. Molarity changes with temperature but molality does not. Molar mass of NaOH is $40$g/mol; it is an inorganic compound which is known as sodium hydroxide or caustic soda. It is soluble in water, ethanol and methanol. It is highly used in manufacturing of soaps, explosives, petroleum products and dyestuffs.