Chemistry

A solution is made by mixing 42.0 mL of ethanol, C2H6O, and 58.0 mL of water. Assuming ideal behavior, what is the vapor pressure of the solution at 20 C in torr?
Values at 20C
ethanol density: 0.789 g/mL & vapor pressure (torr): 43.9 torr
water: denisty: 0.998 g/mL & 17.5 torr

Any steps would be GREATLY appreciated!

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  1. Calculate moles H2O.
    Calculate moles EtOH.
    Calculate XH2O
    Calculate XEtOH.
    Calculate pH2O
    Calculate pEtOH.
    Total P = pH2O + pEtOH

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  2. Hi Dr Bob, is this right so far?
    mass h2o = (d)(v) = (0.998 g/mL)(58 mL) = 57.884 g
    57.884 g H2O/18g H2O = 3.2157 mol H2O
    mass ethanol = (0.789 g/mL)(42 mL) = 33.138 g
    33.138 g C2H6O/46.0687 g C2H6O = 0.719 mol C2H6O

    molality (m) = 0.71937 mol C2H6O/0.057884 kg H2O = 12.43 m

    xH2O = 12.45/(12.43+3.2157) = 0.794
    xEthanol = 12.43/(12.43 + 0.719) = 0.91
    ?

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  3. Hi Dr Bob, is this right so far?
    mass h2o = (d)(v) = (0.998 g/mL)(58 mL) = 57.884 g
    57.884 g H2O/18g H2O = 3.2157 mol H2O
    mass ethanol = (0.789 g/mL)(42 mL) = 33.138 g
    33.138 g C2H6O/46.0687 g C2H6O = 0.719 mol C2H6O

    You are ok to here with moles.
    You don't need molality. You want mole fraction. mole fraction EtOH = XEtOH = moles EtOH/total moles. mole fraction H2O = XH2O = moles H2O/total moles
    Then pH2O = XH2O*PoH2O and
    pEtOH = XEtOH*PoEtOH.
    Total P = pH2O + pEtOH.


    molality (m) = 0.71937 mol C2H6O/0.057884 kg H2O = 12.43 m

    xH2O = 12.45/(12.43+3.2157) = 0.794
    xEthanol = 12.43/(12.43 + 0.719) = 0.91
    ?

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  4. i got it! thanks Dr Bob

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