Question

0. 102 g of an unknown compound dissolved in 100. Ml of water has an osmotic pressure of 28. 1 mmhg at 20°c. Calculate the molar mass of the compound.

Answer 1

The molar mass of the compound is 68 g/mol.

What is the molar mass?

The Osmotic pressure can be obtained from the relationship;

π= iCRT

π= osmotic pressure = 28.1 mmHg or 0.037 atm

i = Vant Hoff factor = 1

C = concentration = ?

R = gas constant = 0.082 atmLK-1Mol

T = temperature = 20°c + 273 = 293 K

C = π/iRT

C =  0.037/1 * 0.082  * 293

C = 0.0015 M

Now;

Number of moles = C/V = 0.0015/100 * 10^-3

= 0.015 moles

Number of moles = mass/molar mass

Molar mass =  mass/Number of moles = 0. 102 g/ 0.015 moles = 68 g/mol

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