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 Molality (m)

Molality is another unit of concentration.

It is similar to Molarity, except instead of per Liter of Solution it is per kilogram of Solvent.

 vs.

At different temperatures, Molarity can change because it is based on volume. Volume can change when temperature changes, so Molarity will change also.

However, molality is based on mass. Mass does not change when temperature changes. Molality is always constant.

Molality Problems

Calculate the molality of a solution 8.53 g benzene, C6H6, dissolved in 20.6 g carbon tetrachloride CCl4.

Benzene is our solute so find the moles= 8.53g C6H6 (1mol/78g)= 0.109 mol C6H6

Carbon Tet. is the solvent convert to kg= 20.6g( 1.000kg/1000.g)=0.0206kg CCl4

 = 0.109 mol C6H6 =0.529 molal _____________________________________________________ 0.0206kg CCl4

Calculate the molality of a solution of 4.40 g NaCl dissolved in 0.214 L of water.

NaCl is our solute so find the moles= 4.40 g NaCl (1mol/58.5g)= 0.0753mol NaCl

Water is the solvent convert to kg (using density)= 0.214L( 1.000kg/1.000L)=0.214kg H2O

 = 0.0753mol NaCl =0.352 molal _____________________________________________________ 0.2147kg H2O

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