Question

What is the molecular weight of a molecule if 86.5 grams of the molecule gives a freezing point of -0.13°C when dissolved in 847 g water?

Answer 1

$M=Km/\Delta t$ , M - molecular weight of dissolved molecules g/mol, K - cryoscopic constant (1,86K/mol for water), m - the mass of dissolved molecules per 1000 g of water, $\Delta t$ - lower crystallization temperature(K).

$\Delta t=t_2-t_1$ , t₁ - freezing point of pure water(0oC);

$\Delta t =$ 0oC-(-0,13oC)=0,13oC=0,13K

$m=$ 86,5g*1000/847g=102,13g

M=1,86K/mol*102,13g/0,13K=1461,24g/mol

Answer:1461,24g/mol.