A 67 cm^3 block of MgCl2 is put into 240 ml of water. Calculate the molality of the solution.?

Question

Density of MgCl2 = 1.56 g /cm^3

D = m / V, where D = density, m = mass, and V = volume

molality = mol solute / kg solvent

moles = mass / Molar mass

Answer 1

there is a problem, you are given the formula MgCl2, but are provided with the density of the hydrate MgCl2.6H2O...

that give three possible situations:

(1) use what you are given 67 cc MgCl2 with the wrong density 1.56g/cc:

find grams, using density:

67 cm^3  @ 1.56g/cc = 104.52 grams MgCl2

find moles, using molar mass:

104.52 grams MgCl2 @ 95.21 g/mol  =  1.098 moles

find molarity:

1.098 moles / 0.240 kg H2O  = 4.57 Molal

first answer(2sig figs): 4.6 Molal

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2) use what you are given 67 cc MgCl2 with its real density of 2.32 g/cm³:

find grams, using density:

67 cm^3  @ 1.56g/cc = 104.52 grams MgCl2.6H2O

find moles, using molar mass:

104.52 grams MgCl2,6H2O @ 203.31 g/mol  =  1.62 moles

find molarity:

1.62 moles / 0.240 kg H2O  = 6.80 Molal

second answer(2sig figs): 6.8 Molal

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3) use what you are given 67 cc as ...MgCl2.6H2O with its density of 1.56 g/cm³:

find grams, using density:

67 cm^3  @ 2.32 g/cm³ = 155.44 grams MgCl2

find molesusing molar mass:

155.44 grams MgCl2 @ 203.31 g/mol  =  0.76 moles

find molarity:

0.76 moles / 0.240 kg H2O  = 3.18 Molal

third answer(2sig figs): 3.2 Molal

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see:   http://en.wikipedia.org/wiki/MgCl2

for formulas, molar masses & densities