Question about molality

[shawny8351]

hi all, i have a quick question from Chemistry.

this is a question from Topscore.

"if you dissolved 20 grams of NaOH in 1000g of water, then what is the molality(m) of the solution?"

(mm of NaOH =40g/mol)

the answer says it's 0.5m.

isn't it 1m because NaOH will dissociate in the water making Na+ and OH-?


thank you.

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[TheWiredNerv]

calculate moles of NaOH: 20 / 40 = 0.5 moles NaOH.
molality = moles / kg solvent = 0.5 / 1 = 0.5 m.
Molality is not affected by colligative properties only vapor pressure, boiling point and freezing point depression and osmotic pressure are.

 

[uvaGirL07]

Topscore is right... it's 0.5m

 

[shawny8351]

thank you very much!