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# When 5g of a noneelectrolyte is added to 25g of water, the new freezing point is -2.5 C. What is the molecular mass of the unknown compound?

Since you listed this as basic chemistry I'm going to assume that you're using the simplified equation

ΔTf = -Kf*Cm*i

You've got values for

ΔTf = -2.5°C

Kf = 1.853 (K*kg)/mol

i = 1 because it is a non-electrolyte, there's no ionization

The only other trick would be to remember that molal concentration (Cm) is moles of solute per kg of solvent, and then that moles of solute is equal to

5g solute*(1 mol/x g solute).

so you end up with

2.5 = 1.853*[(5/x)/(0.025 kg)]

Just solve for x.

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