Solutions

Molality

Molality (m)

A concentration that expresses the

moles of solute in 1 kg of solvent

Molality (m) = moles of solute

1 kg solvent

Units of Molality

2.0 m HCl = 2.0 moles HCl

1 kg solvent

6.0 m HCl = 6.0 moles HCl

1 kg solvent

Molality Calculation

NaOH is used to open stopped sinks, to treat cellulose in the making of nylon, and to

remove potato peels commercially.

If 4.0 g NaOH are used with 500. g of water, what is the molality (m) of the solution?

Calculating Molarity

1) 4.0 g NaOH x 1 mole NaOH = 0.10 mole NaOH 40.0 g NaOH

2) 500. g x 1 kg _ = 0.500 kg water 1000 g

3. 0.10 mole NaOH = 0.20 mole NaOH0.500 kg water 1 kg water

= 0.20 m NaOH

Calculating Molality

A KOH solution contains 400. g water and 2 mole KOH. What is the molality of the solution?

1) 8 m

2) 5 m

3) 2 m Dra

no

Solution

2) 5 m

M = 2 mole KOH = 5 m

0.4 kg water

Dra

no

Calculating Molarity

A glucose solution is made with 2.0 kg water and 72 g glucose (C6H12O6). If glucose

has a molar mass of 180. g/mole, what is the molality of the glucose solution?

1) 0.20 m2) 5.0 m3) 36 m

Solution

A glucose solution is made with 2.0 kg water and 72 g glucose (C6H12O6). If

glucose has a molar mass of 180. g/mole, what is the molality of the glucose solution?

1) 72 g x 1 mole x 1 = 0.20 m

180. g 2.0 kg

Molality & Grams & Volume

What is the molality of a solution prepared by mixing 3.0 moles NaOH with sufficient solvent to make a total mass of 400 g?

1) 12 m

2) 10.7 m

3) 300 m

Solution

2) 3.0 mol x 40 g = 120 g solute 1 mol

mass of solvent = 400 g total – 120 g solute = 280 g solvent

m = 3.0 mol 0.280 kg

m = 10.7 m