Question

How many grams of NaCl must be added to 225.0 g
of H2O at 45 °C to lower the vapor pressure of water
by 2.21 mmHg? The vapor pressure of pure water is
71.93 mmHg at 45 °C.
vp lowering = Xsolute*P<sup>o</sup><sub>H2O</sub>
Solve for Xsolute
mols H2O = grams/molar mass
XNaCl = mols NaCl/(mols NaCl + mols H2O)
You know XNaCl and mols H2O, solve for mols NaCl, then
mols NaCl = grams/molr mass NaCl. You know mols and molar mass NaCl; solve for grams NaCl.
Then remember NaCl has i = 2 for the van't Hoff factor so divide the final mols by 2.

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