Question

25g of a solute of molar mass 250g/molecular is dissolved in 100ml of water to form a. Solution whose density is 1.25 g/ml. The molality is? ( answer =0.8) please solve it.

Answer 1

`"1 m"` is correct answer.

Explanation:

Moles of solute`= "25 g"/"250 g/mol" = "0.1 mol"`

Density of water`= 1\ "g/ml"`

Mass of solvent (water)`= 100 cancel"ml" × 1 "g"/cancel"ml" = 100\ "g" = "0.1 kg"`

`"Molality" = "Moles of solute"/"Mass of solvent (in kg)"`

`"Molality" = (cancel"0.1" "mol")/(cancel"0.1" "kg") = "1 m"`

Molality of solution is `"1 m"`

—————————

The one who got `"0.8 m"` used the wrong formula of Molality as shown below

`"Molality" = "Moles of solute"/"Mass of solution (in kg)"`

Mass of solution`= 100 cancel"ml" × 1.25 "g"/cancel"ml" = 125\ "g" = 0.125\ "kg"`

`"Molality" = "0.1 mol"/"0.125 kg" = "0.8 m"`

This is wrong answer. Molality is moles of solute per `1` kg solvent (not solution).

Answer 2

As written your answer is INCORRECT....

Explanation:

By definition...`"molality"="moles of solute"/"mass of solvent in kg"`

And so we got `((25.0*g)/(250.0*g*mol^-1))/(0.100*kg)=1*mol*kg^-1`

Given the solution density, we might be able to work out the molar concentration, but I would readdress the terms of your question FIRST....