Question

20%(w/v)NaoH solution density =1.2g/ml is equivalent to 1) 1.5 molal 2) 5 molal 3) 2.5 molal 4) 4 molal.
sorry it's chem ques by mistake I chose phy

Answer 1

answer : option (2) 5molal

explanation : 20% (w/V) NaOH solution.it means, 20g of NaOH present in 100ml of solution.

we know, molecular mass of NaOH = 40g/mol

so, no of moles of solute = 20g/40g/mol = 0.5mol

density of solution is given 1.2g/ml

so, mass of 100ml solution = density × volume = 1.2 × 100 = 120g

mass of solvent = mass of solution - mass of solvent

= 120g - 20g = 100g

so, molality = no of moles of solute/mass of solvent in Kg

= 0.5/(100/1000)

= 5 molal