What are the concentrations of each of the following solutions?

a) The molality of a solution prepared by dissolving 25.0g of H2SO4 in 1.30L of water

b) The mole fraction of each component of a solution prepared by dissolving 2.25g of nicotine, C10H14N2, in 80.0 g of CH2Cl2

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a) The molality is defined as follows,

Molality of solute in a solution is,

Molality = number of moles of solute/ mass of solvent.

Remember it is mass of solvent, NOT the solution. So this can be tricky.

Molar weight of H2SO4 = 98 g per mol

Assuming H2SO4 used is 100% pure sample.

Number of moles of H2SO4 = 25 g / 98 g per mol

= 0.255 mol

I assume that density of water is 1000 kg/m3 (1000 g per litre).

The mass of solvent present = 1.3 L x 1000 g/L

= 1300 g.

= 1.3 kg.

Therefore molality of the solution is,

**Molality = 0.255 mols of H2SO4 / 1.3 kg of water**

** = 0.1962 mol/kg**

b) Molar weight of C10H14N2 = (12x10+1x14+14x2)

= 162 g/mol

Molar weight of CH2Cl2 = (12x1+1x2+35.5x2)

= 85 g/mol

Therefore number of C10H14N2 mols = 2.25 g / 162 g/mol

= 0.01389 mol

Number of CH2Cl2 mols = 80 g / 85 g/mol

= 0.94118 mol

Therefore total mols peresent in solution

= 0.01389 mol + 0.94118 mol

= 0.95507 mol

**Mole fraction of C10H14N2 = 0.01389 mol / 0.95507 mol**

** = 0.0145**

**Therefore mole fraction of CH2Cl2 = 1 - 0.0145**

** = 0.9855**

**Sources:**

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