Question

Question 14 A student dissolves 1.5g of styrene C8H8 in 225.mL of a solvent with a density of 1.02/gmL . The student notices that the volume of the solvent does not change when the styrene dissolves in it. Calculate the molarity and molality of the student's solution. Round both of your answers to 2 significant digits. molarity = molality = Ă—10

Answer 1

We must to know:

Cm = molarity = niu / Vs, when the niu = no. of moles and Vs = Volume of solution

the no. niu = mass / molecular mass of substance

molecular mass of C8H8 = 12x8+8x1 = 104 g/mol

=> niu = 1,5 / 104 = 0,0144 moles C8H8

=> Cm = 0,0144/0,225 = 0,06 mol/L

Cmm = molality = niu (C8H8) / mass of solvent (kg)

=> p = mass / V => mass (solvent) = p x V

=> 225 x 1,02 = 229,5 g solvent = 0,2295 kg solvent

=> Cmm = 0,0144 / 0,229,5 = 0,063

Answer 2

Answers:

a. 0.064 mol/L; b. 0.063 mol/kg

Explanation:

a. Molar concentration

c = moles/litres

=====

Moles = 1.5 × 1/104.15

Moles = 0.0144 mol

=====

Litres = 225. × 1/1000

Litres = 0.225. L

=====

c = 0.0144/0.225.

c = 0.064 mol·L⁻¹

===============

b. Molal concentration

b = moles of solute/kilograms of solvent

=====

Mass of solvent = 225. × 1.02/1

Mass of solvent = 229.5 g      Convert to kilograms

Mass of solvent = 0.2295 kg

=====

b = 0.0144/0.2295

b = 0.063 mol/kg