Critical item 12:

1.
Calculate the vapor pressure of water at 25^oC if 10.0 g of ethanol (molar mass = 32 g/mol) is dissolved in 500 g of pure water.  The vapor pressure of pure water at 25^oC is 23.8 torr.
                                                                                                                                                                                          answer
 

2.
How much sodium chloride (formula mass = 58.5 g/mol) is required to lower the vapor pressure of water by 10% if one starts with 500 L of water?
                                                                                                                                                                                        answer
 

3
5.0 g of butanol (C₄H₉OH) is added to 100 g of ethanol (C₂H₅OH) at the boiling point of ethanol.  What is the resultant vapor pressure of the ethanol.
                                                                                                                                                                                       answer
 

1 ANS

n_solute = m / M

n_solute = 10.0 g / 32.0 g mol^-1

n_solute = 0.313 mol

(Caution:  Here the van't Hoff factor is 1 - remember if you have an electrolyte you must multiply by the van't Hoff factor!)

n_solvent = m / M

n_solvent = 500.0 g / 18.02 g mol^-1

n_solvent = 27.75 mol

n_Total = 28.06 mol

X_solvent =  n_solvent / n_Total

X_solvent = 27.75 mol / 28.06 mol

X_solvent = 0.989

P_solvent = X_solventP^o_solvent

P_solvent = 0.989 (23.8 torr)

P_solvent = 23.5 torr

2 ANS 

P_solvent = X_solventP^o_solvent 

since: P_solvent = 0.90 P^o_solvent    then X_solvent= 0.90

n_solvent = m / M

n_solvent = 500.0 g / 18.02 g mol^-1

n_solvent = 27.75 mol

X_solvent =  n_solvent / n_Total

0.90 = 27.75 mol / (27.75 mol + n_{total solute})

n_{total solute} = 3.08 mol

n_NaCl = n_{total solute} / 2

n_NaCl = 1.54 mol

m_NaCl = 90.1 g

3 ANS 

n_solvent = m / M

n_solvent = 100.0 g / 46.0 g mol^-1

n_solvent = 2.17 mol

n_solute = 5.0 g / 74.0 g mol^-1

n_solute = 0.068 mol

X_solvent =  n_solvent / n_Total

X_solvent =  2.17 mol / (2.17 mol + 0.068 mol )

X_solvent =  0.969

P_solvent = X_solventP^o_solvent 

P_solvent = 0.969 x 1.00 atm