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

0.23mol/kg

Step-by-step 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 =
`(number of moles )/(mass of solvent)`

Number of moles of KBr;

Number of moles =
`(mass)/(molar mass)`

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

Number of moles =
`(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 =
`(0.21)/(0.9)` = 0.23mol/kg