Question

Calculate the molality of 25.0 grams of potassium bromide dissolved in 900 mL of pure water. (density of pure water is 1.0 g/mL).​

Answer 1

Answer:

0.23mol/kg

Explanation:

Given parameters:

Mass of Potassium Bromide  = 25g

Volume of pure water = 900mL

Unknown:

Molality = ?

Solution:

Molality is one of the ways of measuring concentration. It is the number of moles of solute in given mass of solvent.

 Mathematically;

        Molality  = $\frac{number of moles }{mass of solvent}$

Number of moles of KBr;

   Number of moles  = $\frac{mass}{molar mass}$  

      Molar mass of KBr = 39 + 80  = 119g/mol

 Number of moles  = $\frac{25}{119}$   = 0.21mole

Mass of solvent;

   Mass of solvent  = density of water x volume of water

   Mass of solvent = 1.0g/mL x 900mL = 900g  = 0.9kg

Now;

 Molality  = $\frac{0.21}{0.9}$   = 0.23mol/kg